An　in　situ　scanning　electron　microscope　(SEM)　tensile　test　for　Ni-based　single-crystal　superalloy　was　carried　out　at　1000　degrees　C.　The　stress　displacement　was　obtained,　and　the　yield　strength　and　tensile　strength　of　the　superalloy　were　699　MPa　and　826　MPa,　respectively.　The　crack　propagation　process,　consisting　of　Model　I　crack　and　crystallographic　shearing　crack,　was　determined.　More　interestingly,　the　crack　propagation　path　and　rate　affected　by　eutectics　was　directly　observed　and　counted.　Results　show　that　the　coalescence　of　the　primary　crack　and　second　microcrack　at　the　interface　of　a　gamma/gamma　＇　matrix　and　eutectics　would　make　the　crack　propagation　rate　increase　from　0.3　mu　m/s　to　0.4　mu　m/s.　On　the　other　hand,　crack　deflection　decreased　the　rate　to　0.05　mu　m/s.　Moreover,　movement　of　dislocations　in　front　of　the　crack　was　also　analyzed　to　explain　the　different　crack　propagation　behavior　in　the　superalloy.