Rock　microstructures,　including　the　mineral　heterogeneity　and　initial　microcracks,　not　only　affect　its　failure　process　but　also　influence　its　nonlinear　mechanical　behavior.　However,　in　previous　studies　the　rock　microstructures　especially　the　width　of　initial　microcracks　are　usually　unreasonably　considered,　which　as　a　result　fail　to　capture　the　rock　realistic　nonlinear　mechanical　behaviors　(such　as　the　gradual　crack　closure　characteristic)　and　failure　process.　In　this　study,　an　image-based　model　considering　the　rock　microstructures　is　developed　by　an　FDEM　scheme.　The　rock　microstructures　are　characterized　by　a　series　of　image　process　and　then　are　incorporated　into　the　numerical　model.　The　width　of　the　initial　microcracks　are　considered　in　the　model　through　inserted　cohesive　elements　with　certain　thickness.　The　proposed　model　is　firstly　validated,　based　on　which　the　influences　of　microcrack　density,　microcrack　width　and　microcrack　orientation　on　rock　mechanical　behavior　are　then　discussed.　The　numerical　results　indicate　that　the　initial　microcrack　has　a　larger　influence　on　the　stress　distribution　than　the　mineral　heterogeneity　and　its　width　is　a　key　factor　affecting　rock　crack　closure　behavior.