Deformation and strength characteristics of granite under true triaxial stress states ( σ 1 ⩾ σ 2 ⩾ σ 3 ) are studied using 3D discrete element method (DEM). Lode angle θ σ and the minimum principal stress σ 3 are kept constant during each test by using distortional periodic boundary, eliminating the end effect and isolating the effect of intermediate principal stress σ 2 . Results reveal significant effect of σ 2 on the common mechanical behavior and magnitude distribution of acoustic emission events. Based on a three-invariant strength criterion, variations of the instantaneous internal friction angle upon failure give an explanation for the σ 2 effect on peak strength, which can be explained additionally by the energy release quantified as b-value.