Background: Considering the EAF（Environment Assistant Fatigue） effect during the design status is a key issue for generation III nuclear power plant.Purpose: The calculation methodology of Fen（Environmentally Assistant Fatigue Factor） and strain ratio of ASME Code Case N-792 has been introduced in this paper.Methods: By creating 2D symmetry model of reactor pressure vessel nozzle

the environmentally assistant fatigue factor Fen and fatigue usage factor has been analyzed according Code Case N-792 with simplified and detailed integral methods.Results: The fatigue usage factor results of ASME-III which does not consider environmental effect and Code Case N-792 which consider the environmental effect have been compared and discussed.Conclusions: And the fatigue life considering Fen effect of SCL1 section will reduce to about 1/3 of the life in air condition while the fatigue life of SCL2 decrease 3/5.