Self-propelled swimmers using ultrasonic transducers have been proposed and studied. The screw and bionic propulsion systems have dominated underwater robots. However, the complex structures make miniaturization difficult. The swimmers with ultrasonic transducers have no moving parts such as fins or propellers and can be driven by the nanomechanical-vibration in the surface of the transducer. Simple structure and high driving frequency make it easy to miniaturize. A 36° Y-cut lithium niobate (LN) thickness-vibration-mode transducer is used to investigate an underwater acoustic propulsion system. LN transducers are manufactured to 10 × 10 mm, 7 × 7 mm, and 4 × 4 mm square plates. The zero-speed propulsion and no-load speed are investigated to evaluate the swimmer performance. A mm-order swimmer is studied with a 4 × 4 mm LN transducer. At last, the multi-degrees-of-freedom swimmer driven by three 7 × 7 mm LN transducers is demonstrated with the advance and sway locomotion. In the future, a small-scale and agile self-propelled swimmer with an acoustic propulsion system can be expected for narrow and complex environment tasks, such as seagrass-prone areas and narrow pipelines. Display omitted •At 10 W, the zero-speed propulsions of LN and PZT transducer are 7.8 mN and 4.9 mN. The zero-speed propulsion of LN transducer is 1.6 times greater.•A mm-order miniaturized swimmer with 4 × 4 × 0.35 mm LN transducer is realized.•A triangle multi-degree-of-freedom swimmer with three 7 × 7 mm LN transducers is designed and fabricated.