You enable Page Output Caching by adding an <%@ OutputCache %> directive to a page. : Page Output Cache « Cache « ASP.NET Tutorial

The following page caches its contents for 15 seconds.

<%@ Page Language="C#" %>
<%@ OutputCache Duration="15" VaryByParam="none" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

    void Page_Load()
    {
        lblTime.Text = DateTime.Now.ToString("T");
    }

</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Cache Page Output</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

    <asp:Label
        id="lblTime"
        Runat="server" />

    </div>
    </form>
</body>
</html>

13.9.Page Output Cache
	13.9.1.	You enable Page Output Caching by adding an <%@ OutputCache %> directive to a page.
	13.9.2.	OutputCache Duration="20" VaryByParam="None"
	13.9.3.	Varying the Output Cache by Parameter
	13.9.4.	Varying the Output Cache by Control
	13.9.5.	Varying the Output Cache by Header
	13.9.6.	Varying the Output Cache by Browser
	13.9.7.	Varying the Output Cache by a Custom Function
	13.9.8.	Manipulating the Page Output Cache Programmatically
	13.9.9.	Creating Page Output Cache Profiles