I found a solution. I had this problem because the radmenu didn't have a fixed width :
pan id="HeaderMenuDiv" width='70%'>
telerik:RadMenu style="z-index:1" ID="RadMenu1" runat="server" Skin="WebBlue"
I changed the width :
<span id="HeaderMenuDiv" width='710px'>
<telerik:RadMenu .... />
It works better now. The menu doesn't diseapper when I resize the browser, and doesn't take several lines when the browser is too small.