Present paper proposes a new algorithm of the integrity evaluation of the pressure vessel of nuclear reactors pressurized thermal shock, PTS. This method enables us to do an effective evaluation by superimposing proposed "PTS state-transient curves" and "toughness transient curves", and is superimposing proposed "PTS state-transient curves" and "toughness transient curves", and is superior to a conventional one in the following points ; (1) easy to get an overall view of the result of PTS event for the variations of several parameters, (2) possible to evaluate a safety margin for irradiation embrittlement, and (3) enable to construct an Expert-friendly evaluation system. In addition, the paper shows that we can execute a safety assurance test by using a flat plate model with the same thickness as that of real plant.