Abstract: Largebreak loss of coolant accident (LBLOCA) is among the limiting design basis accidents (DBAs) that determine operation power of nuclear power plants (NPPs). Based on the best estimate (BE) system code RELAP5/MOD3, the advanced code plus conservative evaluation models (EMs) LOCA analysis tool and method were developed, which modified related models/correlations within the code, and were verified through related separate and integral effect tests. This makes it meet the conservative EM requirements of 10CFR50 Appendix K. With the achieved tool and method, double ended guillotine break LOCA analysis of a 300 MW PWR NPP was carried out. The resulted peak cladding temperature (PCT), as compared with the emergency core cooling system ( ECCS) acceptance criterion and corresponding PCT value in final safety analysis reports (FSAR), indicates that further PCT margin is expected to be potentially acquired with the methodology.