i got the solution for this problem
i m using now java script code
<script type="text/javascript">
function fnClearFileSelection(objFileUpload) {
objFileUpload.value = "";
objFileUpload.focus();
return false;
}
</script>
and the file upload is looking like this
<asp:FileUpload ID="fuPhoto" runat="server" ClientIDMode="Static"/>
and the Clear button looks like this
<asp:LinkButton ID="lnkClear" runat="Server" OnClientClick="return fnClearFileSelection(FuPhoto);"></asp:LinkButton>