diff --git a/zapcore/write_syncer.go b/zapcore/write_syncer.go
index 16e5d5d6b..c397a422a 100644
--- a/zapcore/write_syncer.go
+++ b/zapcore/write_syncer.go
@@ -91,6 +91,8 @@ const defaultBufferSize = 256 * 1024
 const defaultFlushInterval = 30 * time.Second
 
 // Buffer wraps a WriteSyncer in a buffer to improve performance,
+// if bufferSize=0, we set it to defaultBufferSize
+// if flushInterval=0, we set it to defaultFlushInterval
 func Buffer(ws WriteSyncer, bufferSize int, flushInterval time.Duration) WriteSyncer {
 	if lws, ok := ws.(*lockedWriteSyncer); ok {
 		if _, ok := lws.ws.(*bufferWriterSyncer); ok {
@@ -126,6 +128,19 @@ func Buffer(ws WriteSyncer, bufferSize int, flushInterval time.Duration) WriteSy
 }
 
 func (s *bufferWriterSyncer) Write(bs []byte) (int, error) {
+
+	// there are some logic internal for bufio.Writer here:
+	// 1. when the buffer is not enough, log would be written to disk directly
+	// 2. when the buffer is enough, log would not be flushed until the buffer is filled up
+	// this would lead to log spliting, which is not acceptable for log collector
+	// so we need to flush bufferWriter before writing the log into bufferWriter
+	if len(bs) > s.bufferWriter.Available() && s.bufferWriter.Buffered() > 0 {
+		err := s.bufferWriter.Flush()
+		if err != nil {
+			return 0, err
+		}
+	}
+
 	return s.bufferWriter.Write(bs)
 }
 
diff --git a/zapcore/write_syncer_test.go b/zapcore/write_syncer_test.go
index 845635738..257a0eb74 100644
--- a/zapcore/write_syncer_test.go
+++ b/zapcore/write_syncer_test.go
@@ -24,7 +24,6 @@ import (
 	"bytes"
 	"errors"
 	"testing"
-	"time"
 
 	"io"
 
@@ -69,13 +68,23 @@ func TestAddSyncWriter(t *testing.T) {
 func TestBufferWriter(t *testing.T) {
 	// If we pass a plain io.Writer, make sure that we still get a WriteSyncer
 	// with a no-op Sync.
-	buf := &bytes.Buffer{}
-	ws := Buffer(Buffer(AddSync(buf), 0, time.Millisecond), 0, 0)
-	requireWriteWorks(t, ws)
-	assert.Equal(t, "", buf.String(), "Unexpected log calling a no-op Write method.")
-	time.Sleep(2 * time.Millisecond)
-	// assert.NoError(t, ws.Sync(), "Unexpected error calling a no-op Sync method.")
-	assert.Equal(t, "foo", buf.String(), "Unexpected log string")
+	t.Run("default", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		ws := Buffer(Buffer(AddSync(buf), 0, 0), 0, 0)
+		requireWriteWorks(t, ws)
+		assert.Equal(t, "", buf.String(), "Unexpected log calling a no-op Write method.")
+		assert.NoError(t, ws.Sync(), "Unexpected error calling a no-op Sync method.")
+		assert.Equal(t, "foo", buf.String(), "Unexpected log string")
+	})
+
+	t.Run("small buffer", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		ws := Buffer(Buffer(AddSync(buf), 5, 0), 5, 0)
+		requireWriteWorks(t, ws)
+		assert.Equal(t, "", buf.String(), "Unexpected log calling a no-op Write method.")
+		requireWriteWorks(t, ws)
+		assert.Equal(t, "foo", buf.String(), "Unexpected log string")
+	})
 }
 
 func TestNewMultiWriteSyncerWorksForSingleWriter(t *testing.T) {