In　conjunction　with　the　transfer　matrix　techniques　for　modeling　ultrasonic　wave　in　multilayered　composite　medium,　the　spring　model　for　a　weak　interface　and　the　interface　condition　of　the　slip　interface,　we　have　the　generalized　frequency　equations　for　low-order　mode　guided　waves　in　ultra-thin　layered　composites　structure　with　a　weak　or　slip　interface　in　different　depth,　and　the　effect　of　the　depth　of　the　interface　on　the　dispersion　properties　of　low-order　mode　ultrasonic　Lamb　wave　are　analyzed.　The　＂ultra-thin＂　here　means　that　the　thickness　of　the　plate　is　only　a　fraction　of　the　ultrasound　wavelength　so　that　the　echoes　from　the　front　and　the　back　of　the　surface　cannot　be　separated　in　the　time　domain.