Javascript Function:
function isOnlyNumber(field) {
var e = event;
var charCode = e.which || e.keyCode;
if (charCode > 47 && charCode < 58)
{
return true;
}
else{
return false;
}
Use This Function in TextBox onkeypress event as below.
<asp:TextBox ID="txt1" runat="server" CssClass="inputbox" onkeypress="return isOnlyNumber(this);"></asp:TextBox>