Abstract: A stacked three dimensional Laser Doppler Anemometer(LDA)system has been developed on the basis of a one demensional and a two demensional LDA systems.This system was successfully used in the measurement of the rotational flow field inside a small axial fan rotor.The measurements reveal rich and complex flow structures near the blade tip region,in particular fine helical vortex structures are obtained,which prove the system a useful tool in turbomachinery flow measurement.