Affected　by　discontinuities,　the　hydraulic　properties　of　rock　masses　are　characterized　by　significant　scale　dependency　and　anisotropy.　Sampling　a　rock　mass　at　any　scale　smaller　than　the　representative　elementary　volume　(REV)　size　may　result　in　incorrect　characterization　and　property　upscaling.　Here,　a　three-dimensional　discrete　fracture　network　(DFN)　model　was　built　using　the　joint　data　obtained　from　a　dam　site　in　southwest　China.　A　total　of　504　two-dimensional　sub-models　with　sizes　ranging　from　1　m　x　1　m　to　42　m　x　42　m　were　extracted　from　the　DFN　model　and　then　used　as　geometric　models　for　equivalent　permeability　tensor　calculations.　A　series　of　steady-state　seepage　numerical　simulations　were　conducted　for　these　models　using　the　finite　element　method.　We　propose　a　new　method　for　estimating　the　REV　size　of　fractured　rock　masses　based　on　permeability.　This　method　provides　a　reliable　estimate　of　the　REV　size　by　analyzing　the　tensor　characteristic　of　the　directional　permeability,　as　well　as　its　constant　characteristic　beyond　the　REV　size.　We　find　that　the　hydraulic　REV　sizes　in　different　directions　vary　from　6　to　36　m,　with　the　maximum　size　aligning　with　the　average　orientation　of　joint　sets　and　the　minimum　along　the　angle　bisector　of　intersecting　joints.　Additionally,　the　REV　size　is　negatively　correlated　with　the　average　trace　length　of　the　two　intersecting　joint　sets.　We　find　that　the　geometric　REV　size,　determined　by　the　joint　connectivity　and　density,　falls　into　the　range　of　the　hydraulic　REV　size.　The　findings　could　provide　guidance　for　determining　the　threshold　values　of　numerical　rock　mass　models.