Abstract: In this paper, the functional relations between the structural parameters of the grinding chamber and gas flow parameters in a jet mill are established systemtically with the theory of gas dynamics and hydrodynamics. The calculation equation of gas velocity is derived. These results not only agree with the experimental conclusions in literature, but also are supported by the authors experiment using Laser Doppler Anemometry to measure the" velocity field in the jet mill. The theoretical basis and calculation method for designing jet mill and selecting its working conditions are provided and a preliminary study has been done to establish the mathematical model of jet mill on the basis of comminution kinetics.