Abstract: A measurement system has been established to measure the size,concentration and velocity of particles in a horizontal air particle free jet from a nozzle of 42mm ID.The principles of evaluation of the particle size and average velocity,which is based on Mie scattering theory and transit timing technique,is presented in this paper.According to the evaluation,a series of the experiments are made to prove the agreement between the evaluation and experimental results,using the powder of aluminum oxide of 10～105μm.The measurement system has the features of the relatively simple optical system,low cast and high space discriminability.