This paper studies the influence of grain gradation on the undrained mechanical behavior of granular materials. A series of conventional undrained triaxial compression tests is carried out on two different materials (glass balls and Hostun sand), which have high degree of shape self-similarity. For each material, 6 samples with similar relative density and different gradations ( 1.1-20) are prepared, and then the samples are sheared. Test results show that the grain gradation has an obvious impact on the undrained mechanical behavior of granular material: both deviatoric stress level and undrained shear strength of two material samples decreases as the coefficient of uniformity ( ) of the material increases, their variations tend to converge as coefficient of uniformity surpasses 5. Based on the evolution of the value of the second-order work of the granular assembly during undrained triaxial loading, potential instability for the selected granular materials is also analyzed. The results demonstrate a significant influence of the grain gradation on stability: the higher of coefficient of uniformity ( ), the larger of the potential of static liquefaction and the unstabler of materials.