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In order to determine the synthesis conditions of α″-(Fe100−xCox)16N2, FeCo nitrided films were fabricated on MgO single-crystal substrates using reactive sputtering. The thermal stability of the α′FeCo phase was evaluated by measuring the temperature dependence of σs. It was found that: (1) for the as-deposited (Fe100−xCox)-N films, α′FeCo martensite phase with a stoichiometric N content of 11 at% can be formed up to x = 10, and with increasing x, α′FeCo phase with stoichiometric N content is not formed. (2) The phase decomposition temperature, Tpd, of α′-(Fe100−xCox)-N (x = 0–30) phase depends strongly on the Co and N contents of the α′FeCo phase. The Tpd of α′ phase decreases from 200°C (x = 0) to RT (x = 30) with increasing Co and N contents. (3) The formation at RT of stable α″-(Fe100−xCox)16N2 (x = 10–30) phase in the FeCo alloy system is concluded to be fairly difficult.