Abstract: The shear flow behaviors of granular materials exist widely in the natural disasters (such as landslide and debris flow) and the industrial process (such as transportation and pumping of mineralmaterials). Granular materials perform various shear flow states under different volume fractions, shear rates and constraint stresses. The investigation of mechanical characteristics in a shear flow process provides an insight to the mechanism of phase transition of granular materials. In this study, a medium-size annular shear cell was developed to study the shear flow states of granular materials and their transition. The shear stress and the volume dilation rate were measured under various normal stresses and shear rates. The experimental results show that the shear stress and the volume dilation rate increase with increasing shear rate. Both of them increase with the square of shear rate piecewise linearly. The inflection points in the linear relations of the square of shear rate versus the shear stress and the volume dilation rate were obtained at the critical shear rate respectively. With the analysis of effective friction coefficis under various shear rates and inertia numbers, the phase transition process between the slow flow and the rapid flow was discussed. The phase transition occurs at the critical shear rate. Moreover, the shear flow states of granular materials were measured under various normal stresses. The critical shear rate decreases with increasing normal stress. This indicates that the normal stress can boost the phase transition between the slow flow state and the rapid flow state. In rapid shear flow state, the effective friction coefficient is independent of the normal stress. The mechanism of phase transition can be well studied with the experiments of shearing granular under various shear rates and normal stresses.