<asp:ScriptManager ID="ScriptManager1" runat="server" LoadScriptsBeforeUI="false">
<Scripts>
<asp:ScriptReference Assembly="AjaxControlToolkit" Name="AjaxControlToolkit.Common.Common.js" />
<asp:ScriptReference Assembly="AjaxControlToolkit" Name="AjaxControlToolkit.Compat.Timer.Timer.js" />
<asp:ScriptReference Assembly="AjaxControlToolkit" Name="AjaxControlToolkit.Animation.Animations.js" />
</Scripts>
</asp:ScriptManager>
Add the following JavaScript codes into your aspx page.
<script type="text/javascript">
//Get a handle to the animation Target. The target can be anything like a 'div', 'table', etc
var target = $get("animationTarget");
//Create an instance of the FadeAnimation .
var fadeAnimation = new AjaxControlToolkit.Animation.FadeInAnimation();
//Set the Duration
fadeAnimation.set_duration( 1.0 );
//Set the Animation Target as a Sys.UI.Control object
fadeAnimation.set_target( target );
//Set the Maximum Opacity Value
fadeAnimation.setValue( 50 );
//Play the Animation
fadeAnimation.play();
</script>