The　non-perfection　of　contacting　interfaces　causes　the　non-trivial　features　including　the　contact　acoustic　nonlinearity　(CAN)　for　ultrasound　propagating　through　the　interface.　A　simple　model　for　one-dimensional　longitudinal　wave　transmission　through　non-perfect　interfaces　is　proposed,　and　the　transmission　coefficient　is　used　to　show　the　interface　stiffness.　Ultrasonic　transmission　experiments　are　carried　out　to　investigate　the　contact　state　of　the　interface　under　different　pressures,　and　the　power-law　relationship　between　the　pressure　and　the　stiffness　coefficients　is　established　by　best　fitting　the　measured　ultrasound　data.　Results　show　that　it　is　feasible　to　characterize　the　pressure-applied　interfaces　by　using　the　transmission　coefficients　of　the　first　and　second　order　harmonics　in　transmitted　ultrasonic　waves.