Natural fault zones undergo pervasive grain comminution during faulting and shearing, producing progressive changes in fault gouge properties. We simulate the comminution process of quartz gouge using the distinct element method (DEM) to examine the influences of grain comminution and associated dynamic changes in grain characteristics on the frictional and micromechanical behavior of granular shear zones. Rounded and triangular grains are constructed from clusters of circular particles, connected by some breakable bonds, allowing for grain fracture and comminution. DEM experiments are conducted by shearing identical granular assemblages composed of either rounded or triangular grains of different strengths over a range of normal stresses from 5 to 100 MPa. The results show that grain comminution with strain changes the partitioning of different deformation mechanisms, mainly by changing grain shape and size. Grain comminution may decrease or increase gouge strength, depending on the direction and degree of change in grain shape. Increases in grain angularity lead to significant increases in frictional strength of fault gouges, while increases in grain elongation tend to decrease in frictional strength. The final strength results from the competition between strength reduction by fracturing and strength variation by changes in grain shape and grain size distribution. Our simulations also demonstrate that the intensity and probability of grain comminution in narrow grain size gouges is affected by shape, strength (e.g., due to mineralogy), and normal stress.