Abstract: This paper presents a method and results of predicting activation source terms of reactor components for NPP decommissioning and its preliminary verification. Through the modification of the one-group neutron reaction cross sections of ORIGEN2, the source terms from activation of reactor components were calculated using the codes of MCNP/ORIGEN2. The results revealed that there were 6 or 7 nuclides which mainly constitute the source terms and the nuclides and their amounts vary significantly with the material composition and the distance to the reactor core of the components. Specific activity of activation samples in a reactor vessel irradiation surveillance capsule were calculated and compared to the measurement data. The comparison indicated that the values calculated with the modification of cross sections agreed well with the measured data within a difference range of 20% while the values calculated without modification of cross sections agreed badly, which verified the applicability of the calculation method presents by this paper.