A series of the second generation bi-crystal superalloys with specific misorientation grain boundaries (GBs) were prepared by the twin crystal seed method, and then their microstructure of GBs was characterized by means of scanning electron microscopy (SEM), meanwhile their tensile properties were examined comparatively at ambient temperature and 760 ^oC. Therewith, the effect of GBs misorientation on the tensile properties of single crystal superalloy can be elucidated clearly. Results show that, the tensile properties decrease with the increase of misorientation, but there is a difference in the decreasing trend for tensile properties at different temperature. At ambient temperature, the tensile strength continuously decreases and the elongation is unchanged with the increase of GBs misorientation. However, at 760 ^oC, the tensile strength is almost unchanged within misorientation below 8°, and decreases rapidly between 8° and 12°, but the elongation decreases rapidly in misorientation ranges of 0°~4° and 8°~12°. With the increase of GBs misorientation, the tensile fracture mechanism changes from cleavage-like fracture to intergranular fracture at 760 ^oC, but the tensile fracture mechanism is always cleavage fracture at ambient temperature. Finally, an energy model was proposed to qualitatively explain the competitive relationship between the two fracture mechanisms during the tensile fracture process at 760 ^oC.