Post accident core damage assessment methodology is a part of nuclear power plant emergency plan. It is used to assess core damage states and extents after an accident. Core damage states include no core damage, fuel clad failure, fuel overheat and fuel melt. The methodology consists of detailed assessment based on sampling system and primary assessment based on on-line monitoring system. A few aspects in post accident core damage assessment methodology developed by Westinghouse in 1984 were improved, such as introducing a modification factor related to fuel burnup and some other factors related to fission product retention on the surfaces of RCS (reactor coolant system) using research results about severe accidents. The methodology and the developed computer code have already been used in Qinshan NPP (nuclear power plant) and can also be used in other PWR (pressurized water reactor) NPPs.