Some of the factors that influence the mercury capillary pressure data of sandstone and carbonate rocks were investigated. These factors include sample size, existence of epoxy coating, and equilibration time. The experimental results show that mercury injection capillary pressure curves are insensitive to the sample size of sandstone samples. Studies on the selected carbonate rock samples show that the injection capillary pressure curves were independent of the sample size. The injection capillary pressure curves obtained by rapid and slow injection methods were almost identical. However, the withdrawal curves for both methods were different. Slow withdrawal curves resulted in lower residual saturations. Epoxy coating did not affect the capillary pressure curves for sandstone samples tested except at low pressure values where a surface effect appears for uncoated samples.