This study is aimed at making clear the effect of Mn and Cr on the microstructure and toughness of an Fe-Cr-Mn alloy which is considered as one of the candidate alloys for reduced activation materials for the first wall application of the fusion reactor. The microstructures of Fe-12% Cr-(5{similar to}30)% Mn(mass%) alloys after solution treatment at 1373 K for 3.6 ks are markedly varied with Mn contents; {alpha}'(martensite) + {delta}(ferrite) in 5% Mn alloy, {alpha}' + {delta} + {epsilon}(martensite) + {gamma}(austenite) in the 10% Mn alloy, {alpha}' + {epsilon} + {gamma} in 15% Mn alloy, {epsilon} + {gamma} in the 20% Mn alloy, and {epsilon} + {gamma} +{delta} in the 25% Mn alloy, and {gamma} + {delta} in the 30% Mn alloy. It is to be noted that the {delta} phase increases with increasing Mn content when the Fe-12% Cr alloy contains more than 25% Mn, which suggests that Mn plays the role of a ferrite former. In Fe-15% Mn-Cr alloy, the {delta} phase is not observed in the range of Cr contents up to 12%, whereas it is markedly increased with the addition of 16% Cr. C, N and Ni are very helpful in forming the {gamma} phase in these alloys as generally known in Fe-Cr-Ni alloys. The toughness evaluated by the Charpy impact test at 273 K and room temperature is very low in the 5% Mn alloy which consists of the {alpha}' and {delta} phases. It is, however, significantly improved by a small amount of the {gamma} phase and increases with increase of {gamma} phase stability. (author).