Abstract: An effective inner boundary layer separation control method was investigated experi mentally.On studying the influence of grooving treatment on the overall performence of the subsonic diffuser, the mechanisms of boundary layer（B.L） separation phenomenon and of the sep-arationdelay-effect of grooves were specially investigated. It was realized that the B.L separation orocess is closely related to the development and distributions of vorticities' within the B.L, while the groove treatment was aimed at controlling the vortex distributions to delay B. L. separation.The method investigated in the work could be of value to the aspect of choosing an efficient way of controlling diffuser separation, and be an alternative in engineering applica tion.