The effects of Co addition on the structure and the saturation magnetization of Fe16N2 were investigated. Enhancement of the formation of Fe16N2 phase by Co addition was observed. Films with 5-25 at% Co contents deposited at proper conditions exhibit a high saturation magnetization up to the range of 2.5-2.7 T. The saturation magnetization of (Fe, Co) 16N2 alloys phase is expected to be 2.8-2.9 T, which is as large as that of Fe16N2. This demonstrates that proper Co addition does not induce notable changes in the high saturation magnetization of the Fe16N2.