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We studied the actuation, vibration and rotation properties of acoustically-driven surface-macromahcined ultrasonic micromotors with different stator dimensions to achieve optimized performances. The motors have been operating continuously in air for over six months without any visible wear. This very long lifetime operation, indicative of motor reliability, in possible by non-contact operation, where the vibration of the stator, which is driven by a die-attached PZT plate, is acoustically coupled to the rotor without contact. The contact is due to fluidic coupling between the rotor and stator. When the resonant frequency of the ont of the sator's vibration mode is close to PZT plate's thickness mode resonance (~4.2MHz), rotating wave is generated on the stator. The rotational threshold and maximum rotation speed of the motor varies with the size of the stator. While larger stator (820Â¿m) has lower rotation threshold (3.5V), its maximum rotation speed (180 rpm) is significantly lower than smaller stator (740Â¿m), whose maximum rotation speed is over 320 rpm with rotation threshold of 8.5V.