A binary granular mixture (mixture of large and small particles) is characterized by two parameters: the particle size ratio and the percent by weight of small particles. The effects of these two parameters on the void ratio and the shear strength of the granular assembly are examined by direct shear tests on glass beads and 2D DEM simulations. It turns out that the angle of shear resistance (phi) basically increases with decreasing void ratio (e) in every case; this tendency diminishes as the particle size ratio increases. The kinematic constraint mechanism inside the granular materials may be the key of this phenomenon.