Research on irradiation embrittlement of heat affected zone, HAZ, in weldment of RPV was experimentally studied in JAEA. Test pieces simulated HAZ was used in the study, and the result showed no significant difference between the HAZ and base metal in microscopic and mechanical properties. This report shows the result of statistical analysis of HAZ ductile brittle transition temperature shift compared with that shown as an unified equation constructed from a database of surveillance test specimens.