A　Super　Fast　Reactor　is　a　pressure-vessel　type,　fast　spectrum　Supercritical　Water　Reactor　(SCWR)　that　is　presently　researched　in　a　Japanese　project.　One　of　the　most　important　advantages　of　the　Super　Fast　Reactor　is　the　higher　power　density　compared　to　the　thermal　spectrum　SCWR,　which　reduces　the　capital　cost.　A　preliminary　core　has　an　average　power　density　of　158.8W/cc　flj.　In　this　paper,　the　principle　of　improving　the　average　power　density　is　studied　and　the　core　design　is　improved.　After　the　sensitivity　analyses　on　the　fuel　rod　configurations,　the　fuel　assembly　configurations　and　the　core　configurations,　an　improved　core　with　an　average　power　density　of　294.8W/cc　is　designed　by　3-D　neutronic/thermal-hydraulic　coupled　calculations.　This　power　density　is　competitive　with　that　of　typical　Liquid　Metal　Fast　Breeder　Reactors　(LMFBR).　In　order　to　ensure　the　fuel　rod　integrity　of　this　core　design,　the　fuel　rod　behaviors　on　the　normal　operating　condition　are　analyzed　using　FEMAXI-6　code.　The　power　histories　of　each　fuel　rod　are　taken　from　the　neutronics　calculation　results　in　the　core　design.　The　cladding　surface　temperature　histories　are　taken　from　the　thermal-hydraulic　calculation　results　in　the　core　design.　Four　types　of　the　limiting　fuel　rods,　with　the　Maximum　Cladding　Surface　Temperature　(MCST),　Maximum　Power　Peak(MPP),　Maximum　Discharge　Burnup(MDB)　and　Different　Coolant　Flow　Pattern　(DCFP),　are　chosen　to　cover　all　the　fuel　rods　in　the　core.　The　available　design　range　of　the　fuel　rod　design　parameters,　such　as　initial　gas　plenum　pressure,　gas　plenum　position,　gas　plenum　length,　grain　size　and　gap　size,　are　found　out　in　order　to　satisfy　the　following　design　criteria:　(1)　Maximum　fuel　centerline　temperature　should　be　less　than　1900°C.　(2)　Maximum　cladding　stress　in　circumstance　direction　should　be　less　than　lOOMPa.　(3)　Pressure　difference　on　the　cladding　should　be　less　than　1/3　of　buckling　collapse　pressure.　(4)　Cumulative　damage　faction　(CDF)　of　the　cladding　should　be　less　than　1.0.　Finally　the　improved　fuel　rod　design　of　the　new　core　is　proposed.