This paper developed an automatic optical inspection (AOI) technology for characterizing the motion amplitude and the vibration mode shape of in-plane micro-actuators. The technology involves a multi-stage image process method. It includes an image contrast enhancement step and a Canny edge detection method. To evaluate this approach, a rotational electrostatic actuator has been prepared for testing. This actuator performed low contrast optical micrographs under first vibration mode. The testing result shows that the technology can handle low contrast microscopic images quite well. The suggested method successfully provides quantitative information to identify the vibration amplitudes and mode shape of the tested actuator with a space resolution of 0.46μm/pixel.