<script type="text/javascript">
function test()
{
var str="value";
document.getElementById("hidden1").value=str;
}
</script>
<input id="hidden1" type="hidden" runat="server" />
<asp:Button ID="Button1" runat="server" OnClientClick="test()"
Text="Button"
onclick="Button1_Click" />
protected void Button1_Click(object sender, EventArgs e)
{
Response.Write(Hidden1.Value);
}