The formation of σ-phase in a 18% Cr 11% Ni 2.6% Mo 0.8% Ti stainless steel has been investigated. Microscopic and X-ray examinations showed that the formationσf σ-phase from δ-ferrite was accompanied by the formation of austenite, the reaction being most rapid at 850℃The formation of a caused a marked decrease in Izod value and a slight increase in hardness. The σ-phase and its detrimental effects can be removed by a re-solution treatment at 1100℃.