The　mechanism　and　quantitative　descriptions　of　nonlinear　fluid　flow　through　rock　fractures　are　difficult　issues　of　high　concern　in　underground　engineering　fields.　In　order　to　study　the　effects　of　fracture　geometry　and　loading　conditions　on　nonlinear　flow　properties　and　normalized　transmissivity　through　fracture　networks,　stress-dependent　fluid　flow　tests　were　conducted　on　real　rock　fracture　networks　with　different　number　of　intersections　(1,　4,　7,　and　12)　and　subjected　to　various　applied　boundary　loads　(7,　14,　21,　28,　and　35　kN).　For　all　cases,　the　inlet　hydraulic　pressures　ranged　from　0　to　0.6　MPa.　The　test　results　show　that　Forchheimer＇s　law　provides　an　excellent　description　of　the　nonlinear　fluid　flow　in　fracture　networks.　The　linear　coefficient　a　and　nonlinear　coefficient　b　in　Forchheimer＇s　law　J　=　aQ　+　bQ(2)　generally　decrease　with　the　number　of　intersections　but　increase　with　the　boundary　load.　The　relationships　between..　and..　can　be　well　fitted　with　a　power　function.　A　nonlinear　effect　factor　E　=　bq(2)/(aQ　vertical　bar　bQ(2))　was　used　to　quantitatively　characterize　the　nonlinear　behaviors　of　fluid　flow　through　fracture　networks.　By　defining　a　critical　value　of　E　=　10%,　the　critical　hydraulic　gradient　was　calculated.　The　critical　hydraulic　gradient　decreases　with　the　number　of　intersections　due　to　richer　flowing　paths　but　increases　with　the　boundary　load　due　to　fracture　closure.　The　transmissivity　of　fracture　networks　decreases　with　the　hydraulic　gradient,　and　the　variation　process　can　be　estimated　using　an　exponential　function.　A　mathematical　expression　T/T-0　=　1-exp(-alpha　J(-0.45))　for　decreased　normalized　transmissivity　T/T-0　against　the　hydraulic　gradient..　was　established.　When　the　hydraulic　gradient　is　small,　T/T-0　holds　a　constant　value　of　1.0.　With　increasing　hydraulic　gradient,　the　reduction　rate　of　T/T-0　first　increases　and　then　decreases.　The　equivalent　permeability　of　fracture　networks　decreases　with　the　applied　boundary　load,　and　permeability　changes　at　low　load　levels　are　more　sensitive.