Influence of grading on the stress-strain and critical state behavior of granular materials is investigated by employing the three-dimensional discrete-element method (3D-DEM) through conventional drained triaxial tests in compression and extension. It is found that, under the same initial conditions (same e0, p'0), samples with a wider particle distribution exhibit more contractive. For each grading, a unique critical state line is achieved under different stress levels. The results from different gradings highlight that the critical state of the granular samples are significantly affected by the grading: the critical state line shifts down as the coefficient of uniformity (Cu) increases. A relationship between the location of CSL and the grading index Cu is then established. Micromechanical analyses are finally performed to confirm the macro finding of the grading dependency of mechanical behavior of granular materials.