Pitting　corrosion　of　tensile　steel　bars,　usually　causes　the　failure　of　RC　beams　under　the　actions　of　chloride　attack　and　fatigue.　Based　on　the　Fick＇s　2nd　law　and　Paris　law,　a　fatigue　life　prediction　model　for　RC　beams　under　these　combined　actions　is　proposed.　Pitting　corrosion　is　considered　as　a　surface　crack.　Fatigue　life　is　predicted　by　parameters　of　the　service　environment,　service　time　and　fatigue　loading,　which　are　available　data　for　in-service　RC　structures.　The　model　can　produce　reliable　fatigue　life　predictions　according　to　the　validation　results,　and　it　is　proved　that　the　crack　initiation　significantly　affects　the　fatigue　life　due　to　its　large　proportion.　In　addition,　parametric　studies　indicate　that　the　fatigue　life　obviously　decreases　as　the　stress　ratio　decreases　from　0.4　to　0.1,　and　when　the　service　time　increases,　the　fatigue　life　rapidly　decreases　at　first　and　then　gradually　decreases　after　10　years　of　service.　The　fatigue　life　markedly　reduces　as　the　environmental　temperature　(greater　than0　degree)　increases,　but　the　relative　humidity　rarely　affects　it.