In order to extend the life of an artificial hip joint, it is necessary to reduce the amount of UHMWPE cup wear. One of remedies to reduce the wear is to improve its surface roughness and roundness. In the last paper, ultra-precision finishing by means of abrasive waterjet was proposed. Its surface roughness was successfully improved by the method, however, its roundness got worse. In the paper, then, a machining model of the proposed method is developed. The model takes into consideration the machining ability distribution within a jet and effects of stand-off distance and abrasive density upon the machining ability. The machining error of experiments and simulations based on the proposed model in changing the abrasive density has good correspondence qualitatively.