I would like to change the title of the page wich I create with master page but I can't ! How can I do that?  

In your Page write the following code.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        Me.Title = "I am Page"

    End Sub

 protected void Page_Load(object sender, EventArgs e)
    {
       
              Page.Title = "this is title";
    }

Thanks a lot. It really works. But here my question is that search engines can define this titile ? 

I think search engine will define it....

Page.Title = "Your Page Title"; 

it is just rendered as html title tag 

<title>Your page title</title> so search engine will index it