Post your code behind and instead of saying it's not working, tell us what has exactly happened (Didn't get the expected result? Got an unexpected result? Got the same result? An error? Exception?). I've tested this before I posted the reply and it's working for me, so I'm confused why it's NOT WORKING for you. Following is my aspx in case if you are interested

<asp:textbox id="txtTextBox1" Width="170px" Runat="server" TextMode="MultiLine"></asp:textbox>

<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>