I've found the solution ...
the html code would be this :
<ul >
<li><a href="KarEditMyProfile.aspx">aa</a>
</li>
<li ><a href="KarPhoneSearch.aspx">bb</a>
<ul class="sub-menu">
<li "><a href="KarAddPhone.aspx" id="HRef_Add" runat="server">bbbb</a></li>
</ul>
</li>
<li><a href="Karbar.aspx">cc</a>
</li>
</ul>
and in code behind write this :
HRef_Add.Visible = false ;
it works for me