In this study, in order to apply electrolytic in-process dressing (ELID) grinding for surface finishing process of the sliding parts of metal on metal (MOM) artificial hip joints, a new grinding system to grind the inside surface of a hemispherical cup was developed. In this system, the workpiece was pressurized against the centre of the ball shaped grinding wheel with an oscillating pressure pin during the processing. The shape and the radius of the ball shaped grinding wheel were adjusted by using different size abrasive grains. Hemispherical cup-shaped samples of Cobalt-Chromium (Co-Cr) alloy were ground by this grinding system and their surface smoothness was improved. In addition, the roundness of the specimens was the same as or superior to that of the commercially available sliding parts. This result suggests that this grinding system is applicable as a surface finishing process of the sliding parts of MOM artificial hip joints.