SU-8 photoresist has received a lot of attention in the MEMS field because of its excellent lithography properties. However, its high internal stress affects the overall pattern quality of the micro structures. The purpose of this work is to reduce the internal stress in SU-8 micro structure by ultrasonic stress relief technology. The stress relief mechanism of SU-8 micro structure was presented. The effect of ultrasonic stress relief on SU-8 micro structure was studied by experiments. The experimental results show that the internal stress in SU-8 micro structures can be reduced by ultrasonic stress relief technology.