In order to nondestructively characterize the interface roughness of coatings effectively, the ultrasonic reflection coefficient amplitude spectrum (URCAS) involving interface roughness was derived based on the phase-screen approximation theory. The interface roughness was determined by a two-parameter inversion combined with a cross-correlation algorithm. For homogeneous coatings, the effects of ultrasonic wavelength λ, beam coverage, and shape variations of the coating on the roughness measurements were analyzed through numerical calculation. A series of simulations indicated that measurement errors were less than 10% when the relationship between interface roughness and wavelength satisfied Rq=1.5%λ∼11%λ approximately. Ultrasonic experiments were carried out on standard roughness specimens utilizing water immersion, flat transducers. The roughness Rq of the standard roughness specimens were 8.5 μm, 14.2 μm, and 28.6 μm measured by confocal laser scanning microscope (CLSM), respectively. Experimental results show that the roughness of standard roughness specimens obtained by the proposed ultrasonic measurement are in good agreement with the CLSM observations, and the relative errors are less than 8.5%.