We propose the smallest preload generation mechanisms that provide force between the rotor and the one cubic millimeter stator in a micro ultrasonic motor. Miniaturizing the preload mechanism is key for implementation of a micro ultrasonic motor with the best performance into micro devices. In this paper, we evaluate two kinds of preloads that exert in the axial and radial directions: an axial preload mechanism and a radial preload mechanism. The experiments clarify how both preload mechanisms improve motor performance measures such as angular velocity, torque, and efficiency. The micro ultrasonic motors with both preload mechanisms generated torque over 10 plslin at the experimentally optimized preload of approximately 6 g. By designing the miniature preload mechanisms using a micro coil spring, the preload mechanisms are miniaturized with similar motor performance. At present, they represent the smallest and most powerful ultrasonic motors with preload mechanisms. (C) 2017 Elsevier B.V. All rights reserved.