The creep damage in 12Cr1MoV steel has been studied by the positron lifetime measurements. The lifetime spectra were analyzed into three components. The mean lifetime τ

defection lifetime τ2 and its intensity I2 increased with the increasing creep damage rate D. The saturation of positron annihilation parameters was not observed

which was a feature different from the positron studies on deformation and fatigue experiments of metals and alloys. When creep damage rate D> 1%

τ2 and τ3 increased rapidly and I2 reached about 10%. These characteristics strongly suggest that the vacancy clusters were formed from vacancies in the creep process

and finally the voids were formed.