Abstract: The technique of laser Doppler velocity measurement of flow field depends on suitable selection of scattering particles.How to select the suitable scattering particles to measure the flow field with a 3-dimensional particle dynamic analyzer(3D-PDA)has been discussed in this paper.The results show that properties of scattering particles are key factors of effectiilg on the velocity measurement accurate,Thescattering particles should meet the requirements of the small size.low density,high reflective index and suitable concentration .