A Mirau microscopic interferometric system for measuring three-dimensional motions of microelectromechanical systems (MEMS) with nanometer resolution is demonstrated.The system utilizes a commercial Mirau microscopic interferometer,which mounts directly on a light microscope.The system was used for full three dimensional motion measurement of a surface micromachined lateral resonator.In-plane motions were determined from stop-action bright field images taken at the best plane of focus.Out-of-plane motions were determined from stroboscopic interferograms obtained at eight different positions of the objective nanopositioner.Experimental results demonstrate nanometer resolution for both in-plane and out-of plane motions.