Abstract: A definition of the friction factor subjected to oscillating flow through a regenerator in pulse tube cryocooler is obtained by analyzing the momentum equation of a regenerator. The flow characteristics in a regenerator are studied by measuring the instantaneous velocity and pressure drop through mesh matrix on a dynamic experimental apparatus. It is found that the ratio of the friction factors under two kinds of flow (f_inst/f_st) varied with the value of the instantaneous Reynolds numbers in one cycle. The friction factors for increasing f_inst exceed the friction factors for decreasing f_inst corresponding to the same Reynolds number during one cycle, which is a apparent asymmetry. The result could be used to predict the performance of a pulse tube cryocooler and could be helpful to designing a cryogenic regenerator by a numerical simulation model.