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　°C　and　1050　°C　was　studied　by　using　the　in-situ　scanning　electron　microscope　high　temperature　fatigue　test　system.　At　950　°C,　the　alloy　deformation　was　controlled　by　slip　and　the　crack　shear　γ/γ’　phase　cracking,　showing　Stage　I　mode.　At　1050　°C,　the　alloy　crack　tearing　occurred　at　the　γ　phase　parallel　to　the　loading　direction　and　γ/γ＇　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　μm　shows　evident　fluctuations,　which　is　closely　related　to　the　microstructure　and　the　development　stage　of　the　fatigue　crack.　©　2023　Elsevier　Inc.