In some applications, especially for the attitude control of navigation system, the friction moment of instrument ball bearings is one of the much more important factors. Its magnitude and fluctuation affect not only the energy transfer but also the accuracy of signal transfer even though the bearings work at very low speed. Experimental analysis showed that the cage rotated with a minor fluctuation along its orbital axis and even this minor fluctuation could also make a great contribution to the moment for the ball bearing. A simplified model of deformation within the contact of ball and cage was built to deduce the contact time. Combined with the experimental results, the angular acceleration were obtained by the model of contact, then the value of moment due to sliding, which induced the fluctuation of cage, between ball and ribbon cage pockets of miniature bearing were obtained.