Microwaves　penetrate　inside　of　low　loss　dielectric　materials　and　they　are　sensitive　to　the　presence　of　internal　interfaces　and　non-uniformities.　This　allows　microwave　nondestructive　inspection　techniques　to　be　utilized　for　inspecting　dielectric　in　metallic　substrate.　This　article　simulated　Microwave　inspecting　thickness　and　delamination　in　layered-dielectric　in　metallic　substrate,　using　open-ended　rectangular　waveguide　probe.　Effective　reflection　coefficient　of　microwave　is　used　in　the　detection　and　evaluation　to　thickness　or　delimination　in　the　media.　This　paper　optimized　the　detection　frequency　and　standoff,　which　provide　a　reference　for　the　experimental　study.