Abstract: Up till now,in the defect assessment standard of pressure vessel,a semi-experience method,equivalent K（stress intensity factor）conversion,usually used to deal with beried and surface three dimen-sional crack.In this paper,the method using elastic-plastic fracture parameter δ（crack tip opening displace-ment）to replace K（stress intensity factor）is first proposed,and equivalent K conversion in usual pressure vessel assessment is replaced by equivalent δ conversion,and equivalent K conversion and equivalent δ conversion is demonstrated in penny shaped crack.It is observed that the equivalent K conversion is conser-vative,and the equivalent δ conversion is more reasonable than the equivalent K conversion.