I Have Implemented Form Authentication, Its Working Fine,Get Succefully Logout, But When My Welcome Page Get Post Back Once Then, I Click On Logout Button Its Get Logout But, When I Click On...

1.In your Page_Init and Page_Load of your Login.aspx.cs page, used for form authentication add the code like in the next example.

 protected void Page_Init(object sender, EventArgs e)
 {
     if (!IsPostBack)
     {
         //
         // Clear the session cached data
         //
         Session.Clear();
     }
 }

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        FormsAuthentication.SignOut();

    }
    //....
}

2.In your web.config you must set form authentication like in the next example:

<authentication mode="Forms">
  <forms loginUrl="Login.aspx" timeout="2880" />
</authentication>

3.In your master page the LoggedInTemplate section must be similar with the next code:

<LoggedInTemplate>Hello<span class="bold">
                                        <asp:LoginName ID="HeadLoginName" runat="server" />
                                    </span>! [
                                    <asp:LoginStatus ID="HeadLoginStatus" runat="server" LogoutAction="Redirect" LogoutText="Logout"
                                        LogoutPageUrl="~/" />
                                    ]
</LoggedInTemplate>

4.At the logout time, you could delete the browser cache as is indicated in the next article:
http://stackoverflow.com/questions/49547/making-sure-a-web-page-is-not-cached-across-all-browsers[^]