Cast duplex stainless steel (CF3M) which contained 15% ferrite was aged at 500°C for 1000h, and fracture toughness tests were conducted at various loading rates and temperatures. Fracture resistance of the aged material reduced largely compared with that of unaged material. The fracture resistance of the aged material had strain rate and temperature dipendence. Fracture morphology in the aged material also changed by loading rate and test temperature. The role of embrittled frerrite phase on toughness reduction of duplex stainless steel have described qualitatively based on scanning electron microscope observations.