Abstract: By means of macroscopical and microscopical examination,chemical composition analysis,mechanical property measurement and fracture analysis,the two fractured valve discs of reheat main steam valve were analyzed. The results showed that there existed serious banded structure segregation in metallographic structure of valve disc,the δ ferrite distributed by chain at crystal boundary,structure defects greatly increased brittleness of valve disc materials. The valve discs fractured caused by impact load exceeding bear capacity of valve disc.