Abstract: There exists pockmarks - effect during the PC - Curve determination by mercury injection method. It is found that the pore volume between the surfaces of coarse sample and the mercury is the origin of pockmarks -effect. Based on a lot of experiments, we have obtained the range of pockmark - effect values of various reservior sandstones in Henan Oil Field. The action of pockmark -effect on capillary pressure is analyzed and found use in capillary pressur curves.