Abstract: The theoretical calculation and experimental measurement methods for estimation of activity in the decommissioned nuclear reactor pressure vessel were introduced.The physical estimation model was described,and Monte Carlo compute code and ORIGEN2 code were recommended to be employed for the calculation of the neutron flux and activity in the reactor pressure vessel.Two methods commonly used for determining the activity in the reactor pressure vessel were introduced in detail,i.e.,sampling from the reactor pressure vessel and from the irradiation tube.The neutron flux profile was established to predict the activity in the reactor pressure vessel.