The microstructure of solid solution strengthened Hastelloy C-276 alloys before and after ruptured at 650 oC under different tensile stresses was studied by SEM and TEM. The results show that the fracture surface displays typical dimple morphology, with a few intergranular fractures. It can be concluded that the C-276 alloy has very good high-temperature toughness. A few annealing twins and plenty of dislocations exist in the initial microstructure of C-276 alloy. And after persistent tensile experiments, a large number of deformation twins are formed in C-276 crystals. Moreover, many fine and dispersive precipitations are also formed in the grain boundary and intragranular. Therefore, a good high temperature stress rupture performance of the Hastelloy C-276 alloy arises from co-effects of solid solution-strengthening, precipitation- strengthening and possible deformation twin-strengthening.