With the maturing design of micro electrical mechanism systems （MEMS） , the metrology for MEMS is becoming the hot research in the micro system technology. In this paper, the summodified-laplacian （SML） focus measure and depth estimation method are used to measure the tip deflection of a thermally actuated polymer micro robotic actuator. Ten one underwater images of the micro actuator working at 42℃ have been used to illustrate the idea of measuring the tip deflection using optical focus method. The results can be used for metrology of MEMS. Optical focus method is used to measure the tip deflection, which can act as a feedback input to the actuator. Then, an accurate control of the motion of actuator can be achieved.