A laboratory technique has been developed to measure the hysteretic relationship between capillary pressure and fluid saturation for a rough-walled rock fracture under different states of normal stress. The technique incorporates a loading system and a uniquely designed capillary barrier to the nonwetting phase which allows for fracture closure and provides excellent hydraulic connection of the wetting phase in the fracture with external pressure control. The method is applied to a single, rough-walled fracture in limestone using oil and water as the nonwetting and wetting phase, respectively. The measured capillary pressure curves were found to be well represented by a Brooks-Corey porous media capillary pressure function. A distinct entry pressure, giving rise to initial nonwetting phase invasion, was observed in each test. It was found that the inferred aperture distributions became less skewed with increased normal load. These aperture distributions were inferred using one of three conceptual models presented for nonwetting phase invasion of a rough-walled fracture under conditions of main drainage.