In order to simply and accurately measure the realistic motion of balls in a ball bearing, a method called the static oblique coordinates is proposed to measure the 3D motion of balls in an angular contact ball bearing. This measuring method utilizes Hall elements as the sensors which are assembled on the side of outer race and cage of the ball bearing with magnetized balls. The voltage signals of the Hall sensors are collected online and applied to calculate and analyze the variation of magnetic axis of the magnetized ball, from which the 3D motion of the balls can be calculated. The basic principles and experiment method of the measuring method are illustrated. This method has simple structure without damaging the bearing, by which the bearing’s real motion can be studied consequently with a high precision. The magnetization and magnetic pole detection methods in this study effectively solved the problem of magnetized balls’ magnetism un-uniformity which causes the measure signal chaos.