Abstract: 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.