Abstract: This paper describes an investigation into the effect of inter-particle friction on shear behavior of granular materials, by performing a series of direct shear tests on an analogous soil - glass bead. In this test study, four types of inter-particle friction conditions are concerned, including dry state, water-lubricated state, flooded state and oil-lubricated state. An analysis of the test results reveals that oil lubrication is able to significantly decrease dilatancy and shear strength of granular specimens, whereas the means of water lubrication and complete submergence in water do not exert a notable effect. Based on test results, a stress-dilatancy equation is proposed that is able to properly take into account the effect of inter-particle friction on the dilatancy of granular materials. The effect of inter-particle sliding friction angle on critical state friction angle is preliminarily examined from a perspective of microscopic particle motions, namely, particle sliding and rolling.