Abstract: As the precision of the spherical couple parts (i.e. the hemisphere and the ball bowl) of a hemisphere dynamic pressure motor is very high, their ball diameter and sphericity measurement during fabricating process have to rely on high precision three-dimensional and roundness measuring instruments. Therefore the measuring process is tedious, complicated, time-consuming, and inefficient. For a long time, it has been one of the bottlenecks that reduce efficiency in the manufacture of hemisphere dynamic pressure motors. In this paper, a ball diameter measuring instrument developed on the basis of the principal of optical interference is introduced. Its application showed that it featured simple operation, high precision, and visible results. It can be used for in-situ measurement while the parts are under fabrication, which greatly improves the measurement efficiency. It has great potential for increasing the efficiency of manufacture of hemisphere dynamic pressure motors and other spherical parts as well.