© 2016 IEEE. Although the motion error around motion direction changing points of each axis affects the accuracy of machined parts, evaluation and compensation methods for the dynamic behavior around motion direction changes of rotary axis has not been established up to now. In this study, an evaluation method which can evaluate the behavior of rotary axis around motion direction changes by synchronous motion of translational and rotary axes is proposed. In the method, direction of translational axes does not change when the motion direction of a rotary axis changes. Measurement test and actual cutting tests are carried out in order to clarify the influence of the behaviors onto the motion trajectory and machined surface quality, caused by the motion direction changing of the rotary axis. Simulations of the motion are also carried out to discuss the causes of inaccuracy.