A novel Micro Optical Probe Sensor (MOPS) is proposed for dynamic in-plane displacement sensing of small objects subject to high frequency and small amplitude of in-plane vibration. A thin reflection grating with high spatial frequency is attached to or made directly on the object to be sensed. When the micro optical probe sensor (MOPS), which includes diode laser source, optical probe generation system and photodetector, scans the diffraction grating, a sinusoidal output is generated. Each cycle of this sinusoidal waveform corresponds to one pitch of the diffraction grating. By analyzing these output signals, the vibration amplitude and frequency can be determined. The approach is validated by experiments using an Optical Pickup Unit (OPU) as the MOPS probe detector and a piece of CD as the grating element. Experiments use electromagnetic actuator (EMA) and piezo-electric transducer (PET) to generate different vibration amplitude and frequency. The measurement resolution of 0.4 micron and the accuracy of 0.8 micron are achieved. The experiments have also shown the capability of measured vibration frequency greater than 21 kHz and vibration amplitude smaller than 1 micron.