I added Ajax control tool kit in my project but I when i run page i found error like

Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.
Wat should i do ?
Please help me...

To use ajax you need to add ajax to your Asp.Net controls i.e ToolBox from reference and in Your project also.
After in which page you are using ajax just write this line. after declaration of page

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="Default" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

and add ToolKitScripManager or ScriptManger from ajax control in the page you are using ajax..

- If You are using AjaxControlTooklit3.5 you should use ToolkitScriptManager of AjaxControlToolkit instead of asp:ScriptManager

<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" EnablePartialRendering="true" EnableScriptGlobalization="true" EnableScriptLocalization="true" EnablePageMethods="true" runat="server">
- Next Point is, Check .NetFramework3.5Sp1 is installed in your system or not. If not installed download from .NetFramework3.5Sp1 and install.
Problem will be solved.

What r u saying exactly...?
I do not understand.

Your problem is not due to coding error.

Step 1: Install "Microsoft.Net Framework 3.5 Sp1", If You are using AjaxControlToolkit3.5.dll
Step 2: Use ToolkitScriptManager available in Ajax Controls instead of ScriptManager.

Then let me know what happens. Your problem should be solved.

@paresh tomhar it is not important that you use ToolkitScriptManager or ScriptManager it is only used when scriptmanager s not working.
If my answer is not helping than Sharpcp way is also right. You install ASP.NET 3.5 Service Pack 1
and the link to download is

here some videos which help you to download if you face some problems.

"Necessity is the mother of Invention".