Shear　strength　of　geomaterials　including　cohesive　strength　and　frictional　strength　is　significantly　influenced　by　the　hydrostatic　pressure.　The　shear　strength　is　provided　by　cohesive　force,　frictional　force　between　granules　and　the　crush　of　granules.　Roles　of　cohesive　strength　and　frictional　strength　played　under　different　hydrostatic　pressures　were　analyzed,　and　the　shear　strength　property　influenced　by　the　crush　of　granules　was　also　studied,　then　the　mechanism　of　hydrostatic　pressure　effect　was　expounded.　When　the　material　was　under　a　low　hydrostatic　pressure,　the　mutual　movement　of　granules　resulted　in　the　failure　of　geomaterials,　which　exhibited　the　shear　stress　ratio　failure　characteristics.　When　the　material　was　under　a　high　hydrostatic　pressure,　the　crush　of　granules　resulted　in　the　failure　of　geomaterials,　which　exhibited　the　shear　stress　failure　characteristics.　The　failure　function　in　meridian　plane　of　triaxial　compression　was　used　to　revise　the　nonlinear　unified　strength　model.　The　revised　nonlinear　unified　strength　model　described　not　only　the　effect　of　intermediate　principle　stress,　but　also　the　effect　of　hydrostatic　pressure　on　geomaterials　more　reasonably.　Compared　with　the　data　from　a　number　of　true　triaxial　tests,　the　revised　nonlinear　unified　strength　model　was　shown　to　describe　well　the　hydrostatic　pressure　effect　and　3D　nonlinear　strength　properties　under　multiaxial　stress　conditions　of　different　materials.　©,　2015,　Academia　Sinica.　All　right　reserved.