I can not handle it. should have used a procedure to complete its work. I logout URL navigate directly to the parser from the website. 
Code:

private void btnlogout_Click(object sender, RoutedEventArgs e)
{
    string logout = Showalltopic.logoutString;
    webBrowser11.Source = new Uri("http://mydomain.com/login.php?do=logout&logouthash="+logout);
    webBrowser11.LoadCompleted += new System.Windows.Navigation.LoadCompletedEventHandler(webBrowser11_LoadCompleted);
    btnlogin.Visibility = System.Windows.Visibility.Visible;
    btnlogout.Visibility = System.Windows.Visibility.Collapsed;
}

void webBrowser11_LoadCompleted(object sender, System.Windows.Navigation.NavigationEventArgs e)
{
    string content = webBrowser11.SaveToString();
    if (content.Contains("All cookies cleared!"))
    {
        MessageBox.Show("Logout success");
        webBrowser11.Navigate(new Uri("http://mydomain.com/", UriKind.Absolute));
    }
    System.IO.IsolatedStorage.IsolatedStorageSettings _appSetting =
        System.IO.IsolatedStorage.IsolatedStorageSettings.ApplicationSettings;
    _appSetting.Clear();
    _appSetting.Save();
}

after logout, WP7 information is stored in isolateStoraged clear that we must be able to logout successfully.
Hope everyone is no other way to clear the cookie in webbrowser.