diff --git a/src/main/java/com/jcraft/jsch/ChannelSftp.java b/src/main/java/com/jcraft/jsch/ChannelSftp.java
index 0711f9e..854cfbf 100644
--- a/src/main/java/com/jcraft/jsch/ChannelSftp.java
+++ b/src/main/java/com/jcraft/jsch/ChannelSftp.java
@@ -803,6 +803,9 @@ public void write(byte[] d, int s, int len) throws java.io.IOException{
           try{
             int _len=len;
             while(_len>0){
+	          if(rwsize<21+handle.length+_len+4) {
+	        	flush();
+	          }
               int sent=sendWRITE(handle, _offset[0], d, s, _len);
               writecount++;
               _offset[0]+=sent;