Fe–N films with 20 nm thick Fe underlayers were deposited on Si(100) substrates by RF magnetron sputtering, and their structure and magnetic properties were investigated. It was found that the optimum nitrogen partial pressure for α'' phase formation is about 2 × 10−4 Torr, and the saturation magnetization measured at room temperature of the Fe–N films deposited at this pressure is as high as 2179 emu cc−1, which is much higher than that of pure iron. Film thickness dependence of the saturation magnetization shows that it initially increases with the increase in film thickness and reaches a maximum of ~2179 emu cc−1 in the thickness range of 300–570 nm, and decreases with a further increase in film thickness.