In the present article, starting from microscopic viewpoint, a new theory of rarefied gas dynamics in the range of free molecular flow has been studied. In this theory, the matrix‐probability method Y. Wu,RarefiedGasDynamics, edited by L. Talbot (Academic, New York, 1961), p. 141; J. Vac. Sci. Instrum.42, 876 (1965) has been employed to calculate transmission probability and a three‐dimension dynamic model has been suggested which can satisfy any complicated boundary conditions theoretically. Finally, the reasons why the transmission probability of gas molecules through a moving channel tends to a constant value (which is smaller than 1) rather than unity while the drag speed increases infinitely have been explained.