As　the　blade　material　of　engine,　nickel-base　single　crystal　superalloy　has　been　subjected　to　complex　service　environment　such　as　high　temperature　and　alternating　load　for　a　long　time.　In　this　work,　the　mechanism　of　fatigue　microcrack　initiation　and　propagation　in　a　nickel-based　single　crystal　at　950　degrees　C　and　1050　degrees　C　was　studied　by　using　the　in-situ　scanning　electron　microscope　high　temperature　fatigue　test　system.　At　950　degrees　C,　the　alloy　deformation　was　controlled　by　slip　and　the　crack　shear　gamma/gamma＇　phase　cracking,　showing　Stage　I　mode.　At　1050　degrees　C,　the　alloy　crack　tearing　occurred　at　the　gamma　phase　parallel　to　the　loading　direction　and　gamma/gamma＇　interface　perpendicular　to　the　loading　direction　and　presents　a　Stage　II　mode　perpendicular　to　the　loading　axis,　then　changes　into　Stage　I　in　the　accelerating　stage.　The　deformation　mechanism　of　fatigue　cracking　at　two　temperatures　was　compared.　Besides,　the　influence　of　the　microstructure　defects　on　the　fatigue　crack　behavior　was　discussed.　The　results　show　that　the　propagation　of　the　microcrack　below　100　mu　m　shows　evident　fluctuations,　which　is　closely　related　to　the　microstructure　and　the　development　stage　of　the　fatigue　crack.