The optically excited vibrations of silicon cantilever resonators are studied. We give the theoretical results and experimental results of cantilever′s coherent frequency and discuss the effect of excited point on the vibration of the cantilever. The amplitude of output signal is related with the position of optical excited point and the vibration mode. We can get the highest amplitude by selecting the center point of two nodal of the vibration mode, then the amplitude decreases with the optical beam move to the nodal of the mode, where we get the lower signal. This result is satisfied to other micromechanical resonators with point heating excitation.