Critical motion error sources of rotary axes with respect to linear axes in 5-axis machining center (MC) include: 1) the alignment error of rotary axes, and 2) the angular error due to e.g. the inclination of rotary axes. Each motion error has a particular "sensitive" direction where it causes the largest motion error in DBB measurement plane. In this paper, the diagnosis method for the motion error sources of rotary axes is proposed as: the first is to find out such the sensitive direction of each motion error source by using numerical simulation, and the second is to determine the diagnosis procedure according to the simulation results. Moreover, the applicability and effectiveness of the proposed diagnosis method is validated by this experiment.