The result of study demonstrated that the addition of Ru changed the damage mechanism of nickel-based single crystal superalloy under thermomechanical fatigue. The formation of deformation twin was suppressed by the addition of Ru. And the main damage mechanism in the Ru-doped alloy was the propagation of crack caused by oxide cracking, while the damage mechanism in the Ru-free alloy was the composite effect of oxidization, deformation twins and even recrystallization.