Influence of nitrogen fractions [Nf=N2/(N2+Ar)] and sputtering powers (Ps) on the structural and magnetic properties of Co–N thin films synthesized by direct current magnetron sputtering have been studied. With increasing Nf from 0 to 20%, a series of phases from β-Co, β-Co (N), Co4N to Co3N were obtained. However, when Nf was fixed at 10%, only Co4N phase with different Co contents in the films was prepared, whose values of saturation magnetism (Ms) increased from 12.9±8.2 Am2/kg to 103.9±6.1 Am2/kg with the increase of Ps. Interstitial nitrogen caused the decrease of coercivity from 24.12 kAm−1 (for β-Co film) to 2.71 kAm−1. However, the addition of interstitial nitrogen was not observed to increase the Ms of β-Co.