As I thought that was my mistake initially, however I found this problem is also existing in the sample site. Anybody noticed that there is a "invisible" link just above the "Home" link. And the value of the link is various depending on which is the current page. it is always the string in the address of browser plus #ct100_MyMenu_SkipLink.

Could anybody give an explanation and the way to solve.

Thanks.

Find the answer myself...

Set the attribute of Menu control, SkipLinkText, to a empty string.