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The stress rupture performances of a directionally solidified Ni-base superalloy DZ951 have been investigated over a wide stress range of 50 to 970 MPa at high temperatures (650 to 1040 ℃). The fracture surface and deformation microstructure were examined by SEM. The results show that the stress rupture strength of DZ951 alloy is much higher than that of DZ40M and almost the same as that of DZ17G alloy. It is suggested that γ′ particles remain cubic shape below 800 ℃, and when the test temperature reaches 900℃, γ′ particles form fine rafted structure. At low temperature (<760 ℃), the fracture mode is cleavage-like. At intermediate temperature (760~850 ℃), the fracture mode shows both cleavage feature and quasi-cleavage feature, and cracks can initiate at the carbide/matrix interface. At high temperature (>850 ℃), the fracture mode turns to be microviod coalessence fracture and the alloy is sensitive to the shrinkage porosity