The relaxation behavior of the compressive stress in nickel-based alloy 600 introduced by WJP and buffing stress improving treatments has been investigated. Hardness reduction behavior in WJP and buffing processed samples during thermal aging has been found to follow the Johnson-Mehl-Avrami equation and the rate constant for hardness reduction to be expressed by the Arrhenius equation. EBSD crystal orientation analysis has confirmed that the strain is introduced mainly in the surface region. Positron lifetime analysis has revealed that the positron lifetime of the WJP and buffing processed samples is longer than that of solution heat-treated sample, indicating the lattice defects have been introduced in the stress improvement process. In addition, it becomes apparent that a large amount of vacancies introduced have been annihilated after thermal aging, however some part of dislocations have still remained. Based the results, the mechanism for stress relaxation behavior of WJP and buffing stress improving treatments has been discussed.