The shear behavior of granular material is not only the most important mechanical property but also the most basic technical indicator in an engineering project. Today, it is still widely used, owing to its simplicity in operation. However, unlike common solid material, granular material shows the properties of disorder and non-uniformity. This is due to the formation of complex network force chains in granular medium. This also leads to the size effect of shear stress in granular solid. In this paper, direct shear tests with various sizes of high-precision spherical glass beads and sample aspect ratio are carried out to investigate the size effect of shear stress. It is found that when the particle size of glass bead decreases, or when the number of glass beads increases, the shear stress decreases slightly, while the reduction of the sample aspect ratio will lead to similar changes. The experimental results show that when the sample length is less than 50 times the particle size or aspect ratio less than 0.5, direct shear tests shows a significant size effect. Therefore the result of this investigation suggests that the guidance for current standard direct shear test should be revised.