Ultrasonic treatment was originally introduced into UV-LIGA technology in this paper. The effect of ultrasonic treatment on SU-8 swelling was researched, and ultrasonic mechanism of SU-8 swelling was explored. Then, a novel method to reduce SU-8 swelling and improve the dimensional precision of an electroformed microstructure was obtained. In experiments, the effect of ultrasonic treatment on the SU-8 swelling during the development and electroforming process was respectively studied, the surface hydrophilicity of SU-8 photoresist in different ultrasonic time was analyzed, and the SU-8 swelling removal ratio in different ultrasonic time was calculated. Furthermore, the effect of ultrasonic treatment on the dimensional precisions of different micro devices was discussed. The experimental results indicate that the SU-8 mould swelling in development process is not obvious, and the ultrasonic treatment has a little effect on the SU-8 mould swelling during development process. The effect of ultrasonic treatment on the SU-8 mould swelling mainly occurs in the electroforming process, and the SU-8 swelling and its surface hydrophilicity both decrease first and increase afterwards with increasing the ultrasonic time. By 10 min ultrasonic treatment, the SU-8 swelling removal ratio is up to 70%, and the dimensional error of electroformed microstructure is independent on the structure of SU-8 mould. Moreover, the reason that SU-8 swelling behavior varies with increasing ultrasonic time was explained based on the ultrasonic mechanical scission of polymer chain and water absorbing mechanism. In conclusion, the presented method to reduce the swelling of SU-8 resist mould does not depend on the process parameters and not increase the complexities of mask layouts, and is a practical method.