diff --git a/pkg/reconciler/vertex/scaling/scaling.go b/pkg/reconciler/vertex/scaling/scaling.go
index 4f165a20ff..951466184e 100644
--- a/pkg/reconciler/vertex/scaling/scaling.go
+++ b/pkg/reconciler/vertex/scaling/scaling.go
@@ -333,21 +333,18 @@ func (s *Scaler) scaleOneVertex(ctx context.Context, key string, worker int) err
 }
 
 func (s *Scaler) desiredReplicas(ctx context.Context, vertex *dfv1.Vertex, partitionProcessingRate []float64, partitionPending []int64, partitionBufferLengths []int64, partitionAvailableBufferLengths []int64) int32 {
-	maxDesired := int32(0)
+	maxDesired := int32(1)
 	// We calculate the max desired replicas based on the pending messages and processing rate for each partition.
 	for i := 0; i < len(partitionPending); i++ {
 		rate := partitionProcessingRate[i]
 		pending := partitionPending[i]
+		var desired int32
 		if pending == 0 || rate == 0 {
 			// Pending is 0 and rate is not 0, or rate is 0 and pending is not 0, we don't do anything.
 			// Technically this would not happen because the pending includes ackpending, which means rate and pending are either both 0, or both > 0.
 			// But we still keep this check here for safety.
-			if maxDesired < int32(vertex.Status.Replicas) {
-				maxDesired = int32(vertex.Status.Replicas)
-			}
 			continue
 		}
-		var desired int32
 		if vertex.IsASource() {
 			// For sources, we calculate the time of finishing processing the pending messages,
 			// and then we know how many replicas are needed to get them done in target seconds.
diff --git a/pkg/reconciler/vertex/scaling/scaling_test.go b/pkg/reconciler/vertex/scaling/scaling_test.go
index 7adaaf40f0..f3fea1af03 100644
--- a/pkg/reconciler/vertex/scaling/scaling_test.go
+++ b/pkg/reconciler/vertex/scaling/scaling_test.go
@@ -38,7 +38,7 @@ func Test_BasicOperations(t *testing.T) {
 	assert.False(t, s.Contains("key1"))
 }
 
-func Test_desiredReplicas(t *testing.T) {
+func Test_desiredReplicasSinglePartition(t *testing.T) {
 	cl := fake.NewClientBuilder().Build()
 	s := NewScaler(cl)
 	one := uint32(1)
@@ -58,14 +58,14 @@ func Test_desiredReplicas(t *testing.T) {
 			Replicas: uint32(2),
 		},
 	}
-	assert.Equal(t, int32(2), s.desiredReplicas(context.TODO(), src, []float64{0}, []int64{0}, []int64{10000}, []int64{5000}))
+	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), src, []float64{0}, []int64{0}, []int64{10000}, []int64{5000}))
 	assert.Equal(t, int32(8), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{10010}, []int64{30000}, []int64{20000}))
 	assert.Equal(t, int32(8), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{9950}, []int64{30000}, []int64{20000}))
 	assert.Equal(t, int32(7), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{8751}, []int64{30000}, []int64{20000}))
 	assert.Equal(t, int32(7), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{8749}, []int64{30000}, []int64{20000}))
-	assert.Equal(t, int32(2), s.desiredReplicas(context.TODO(), src, []float64{0}, []int64{9950}, []int64{30000}, []int64{20000}))
+	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), src, []float64{0}, []int64{9950}, []int64{30000}, []int64{20000}))
 	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{2}, []int64{30000}, []int64{20000}))
-	assert.Equal(t, int32(2), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{0}, []int64{30000}, []int64{20000}))
+	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), src, []float64{2500}, []int64{0}, []int64{30000}, []int64{20000}))
 
 	udf := &dfv1.Vertex{
 		Spec: dfv1.VertexSpec{
@@ -78,7 +78,7 @@ func Test_desiredReplicas(t *testing.T) {
 			Replicas: uint32(2),
 		},
 	}
-	assert.Equal(t, int32(2), s.desiredReplicas(context.TODO(), udf, []float64{0}, []int64{0}, []int64{10000}, []int64{5000}))
+	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), udf, []float64{0}, []int64{0}, []int64{10000}, []int64{5000}))
 	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), udf, []float64{250}, []int64{10000}, []int64{20000}, []int64{5000}))
 	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), udf, []float64{250}, []int64{10000}, []int64{20000}, []int64{6000}))
 	assert.Equal(t, int32(2), s.desiredReplicas(context.TODO(), udf, []float64{250}, []int64{10000}, []int64{20000}, []int64{7500}))
@@ -87,3 +87,25 @@ func Test_desiredReplicas(t *testing.T) {
 	assert.Equal(t, int32(3), s.desiredReplicas(context.TODO(), udf, []float64{250}, []int64{10000}, []int64{20000}, []int64{12500}))
 	assert.Equal(t, int32(3), s.desiredReplicas(context.TODO(), udf, []float64{250}, []int64{10000}, []int64{20000}, []int64{12550}))
 }
+
+func Test_desiredReplicasMultiplePartitions(t *testing.T) {
+	cl := fake.NewClientBuilder().Build()
+	s := NewScaler(cl)
+	udf := &dfv1.Vertex{
+		Spec: dfv1.VertexSpec{
+			Replicas: pointer.Int32(2),
+			AbstractVertex: dfv1.AbstractVertex{
+				UDF: &dfv1.UDF{},
+			},
+		},
+		Status: dfv1.VertexStatus{
+			Replicas: uint32(2),
+		},
+	}
+
+	assert.Equal(t, int32(1), s.desiredReplicas(context.TODO(), udf, []float64{0, 0, 1}, []int64{0, 0, 1}, []int64{24000, 24000, 24000}, []int64{15000, 15000, 15000}))
+	assert.Equal(t, int32(2), s.desiredReplicas(context.TODO(), udf, []float64{5000, 3000, 5000}, []int64{0, 10000, 1}, []int64{24000, 24000, 24000}, []int64{15000, 15000, 15000}))
+	assert.Equal(t, int32(30), s.desiredReplicas(context.TODO(), udf, []float64{5000, 3000, 5000}, []int64{0, 23000, 1}, []int64{24000, 24000, 24000}, []int64{15000, 15000, 15000}))
+	assert.Equal(t, int32(4), s.desiredReplicas(context.TODO(), udf, []float64{5000, 3000, 5000}, []int64{0, 30000, 1}, []int64{24000, 24000, 24000}, []int64{15000, 15000, 15000}))
+	assert.Equal(t, int32(4), s.desiredReplicas(context.TODO(), udf, []float64{1000, 3000, 1000}, []int64{0, 27000, 3000}, []int64{24000, 24000, 24000}, []int64{15000, 15000, 15000}))
+}