I am having the fallowing requeirement :

             <form runat="server>

                <form></form>

                  <%=GetChartsData()%>                 

            <asp:Button id="btn" runat="server" onclick="btn_Click" />

             </form>

      But when i am clicking the button it is not calling the btn_Click event.

Please could you tell me what may be the issue.

try removing the nested <form></form> once..and you will see that the button is working....
I fgure out that you might be using this as suggested by Adobe's http://www.adobe.com/go/kb400730, but trust me you can use the other way around... registering  after the chart is rendered:

<script language="java script">
if(!document.embeds[chartId])
      window[chartId]=document.getElementById(chartId); //or else document.forms[formName/formIndex][chartId]
</script>

You may need to modify the FusionCharts.js 's a code part :

infosoftglobal.FusionChartsUtil.getChartObject = function(id)
{
//  if (window.document[id]) {
//     return window.document[id];
//  }
  if (navigator.appName.indexOf("Microsoft Internet")==-1) {
    if (document.embeds && document.embeds[id])
      return document.embeds[id];
  } else {
    return document.getElementById(id);
  }
}

This works...

Regards,

Sudipto Choudhury
FusionCharts Team