The ball bar is widely used in machine tool measurement because it is convenient and cost effective. Eccentricity exists due to machine errors and installation errors during a ball bar test, and the coordinate transformation method is used to remove eccentricity. However, distortion of data (i.e., ovalization) becomes large when eccentricity relative to the radius of a circle cannot be ignored. The number of sampled data for the total angle of circular path is used to estimate the actual rotating angle of the ball bar during measurement. To prevent distortion of data, actual rotating angles of the ball bar must be estimated exactly. In this paper, geometric relations of the ball bar measurement model are described based on the poses of two reference coordinate systems (ball bar and nominal coordinate system). The proposed algorithm calculates the actual rotating angle of the ball bar using geometric conditions and removes eccentricity through coordinate transformation.