Microelectromechanical systems (MEMSs) have been growing rapidly over recent years. However, measurement of their motions is a difficult problem. A laser Doppler interferometric system for measuring the instantaneous velocity and displacement of the periodic and nonperiodical in-plane and out-of-plane motions of the vibrating combs simultaneously has been developed. The working principle and optical diagram of the system are explained. The instantaneous velocity and displacement of vibrating combs are acquired by demodulating the frequency modulated Doppler signal by using a phase-locked loop. The developed laser Doppler interferometer provides a powerful means in studying the performance of the MEMS devices, particularly in studying their vibration modals. Modal analyses of the vibrating combs are carried out theoretically and experimentally. Due to the nonrigid structure of the MEMS resonator besides the basic modal there are also high order modals in the MEMS vibration. The basic modal is the desired in-plane lateral vibration. The second order vibration is a torsion vibration about the axis perpendicular to the fingers of the movable combs. Both third and fourth order vibrations are lateral bending of the supporting beams. The measured experimental data are presented. The accuracy of the developed interferometer has been checked by an acceleration calibrator.