The recent development of the elastic wave theory for a cracked porous medium significantly enhances our ability to model and predict acoustic response of real rocks. As an important application and verification of the theory, we apply it to model and interpret laboratory ultrasonic velocity data of rock samples measured under pressure loading conditions. The important model parameters, such as crack density and aspect ratio, are obtained by inverting the velocity versus pressure data. The results show that the theory can well describe the elastic velocity variation with pressure under dry and saturated conditions. The theory is applicable for both porous sandstone and very tight granite. The cause of the velocity variation with pressure is the closure of microcracks and the decreasing of crack density within the rock under pressure loading. This paper also suggests using crack density as an important rock property parameter and provides a method for determining the crack parameter.