Structures of existing rotary ultrasonic motors are mostly cylindrical， and pre-pressure is mostly applied by the spring and the nut， which makes the axial length of the motor longer， and is not conducive to connecting with other driving devices. Aiming at the problem， a double-legged micro-rotary ultrasonic motor based on in-plane mode is proposed. The motor consists of a stator and a conical rotor. The stator adopts a circular ring and a double-legged structure. The longitudinal and bending vibration piezoelectric ceramics are attached on the outer side of the legs， and four connecting ears are arranged inside the legs. Silicone rings and an E-type clasp are used between the stator and rotor for preloading and end tightening. The rotor is driven by in-plane traveling waves which travel circumferentially inside the ring of the stator. The stator is simulated with finite element analysis （FEA） software ANSYS， the clamping position and the placement of piezoelectric ceramics in a stator can be determined by studying the influence of the structure on the modal. Sensitivity analysis of the stator modal frequency can be used to guide the modification of the stator structure size （design variables） and improve the optimization efficiency. Three types of prototypes are fabricated and experimentally investigated. Experimental results of mechanical output performance indicate that the motor has a small size with a maximum width of 12 mm， a compact structure and a total weight of 6.9 g. Also， it is easy to connect with other driving devices. The maximum no-load speed is 338 r/min， and the maximum output torque is 1.44 mN·m.