An experiment based on a new optical heterodyne interferometer with microscopic probe has been developed in order to measure the fluid velocity in a micro-channel. By implementing laser Doppler anemometry (LDA) into a microscope, the laser beams are transmitted and then focused on the spot with an extremely highly spatial resolution over an area in 10 μ m diameter. The laser beams are modulated by a diffractive grating in sinusoidal backward and forward movement using a PZT stack, and the standard heterodyne signal, therefore, is recovered by a new demodulation system. In this study, nano-size of particles in 75 μm inner diameter of a micro-channel is driven by a LC pumping. As a result, the trace of velocity of particles can be recorded by a new optical heterodyne microscope. As compared to the other existing heterodyne techniques applied in the fluid velocity measurements, this new technique shows a better frequency response, a wider modulation frequency and an easier and cheaper setup. This setup, however, is limited by the performance of a PZT stack that is used to modulate a diffractive grating. In conclusion, a laser Doppler microscope implemented by a new heterodyne technique is a practical instrument for the micro fluid investigations, especially in MEMS and biological researches.