Screening potential shale gas systems and successful hydraulic stimulation of shale gas plays require careful evaluation of shale mechanical properties and fracability. However, the competitive control of grain size versus content of brittle minerals on shale mechanical properties and fracture behavior is not well understood. We have used the discrete element method to simulate unconfined rock tests on digital shale samples with varying contents and grain sizes of brittle minerals and found that both factors can significantly affect shale mechanical properties and fracture behavior. The content and size of brittle minerals both exhibit positive correlations to the Young’s modulus and unconfined compressive strength of the shale samples, whereas brittle mineral content plays a dominant role. Furthermore, larger sizes of brittle mineral grains resulted in faster fracture generation rates. Our study reveals a combined and also competitive control of the content and grain size of brittle minerals on shale mechanical properties and fracture behavior and points out the critical role of brittle mineral size in shale fracability evaluation, which is believed to deserve more attention during shale gas development.