<head runat="server">
<title></title>
<script>
function clearForm() {
document.getElementById('<%=TextBox1.ClientID%>').value = "";
}
</script>
</head>
<body onload="clearForm()">
<form id="form1" runat="server">
<div>
<asp:TextBox ID="TextBox1" runat="server" >
</asp:TextBox><asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</div>
</form>
</body>