Thank You Pankaj..
Completed code is as follows..
//================in .aspx page=========================
<asp:Button ID="Button2" runat="server" Text="Show Custome-Popup" onclick="Button2_Click" />
<div align ="center" id="popup1" style ="z-index:10000; background-color :Fuchsia; width :265px; height :183px; display :none;">
popup message
</div>
//==============Code Behind======================
protected void Button2_Click(object sender, EventArgs e)
{
string javaScript = "<script language=JavaScript>\n" +
"var obj = document.getElementById('popup1');\n" +
"obj.style.position = 'absolute';\n" +
"obj.style.top = '100px'; \n" +
"obj.style.left = '500px'; \n" +
"obj.style.display = 'block';\n" +
"</script>";
ClientScript.RegisterStartupScript(this.GetType(), "Button2_Click", javaScript);
}