Using the full-potential linearized augmented plane wave method, we report the electronic structures and magnetism in N-doped CdO system. The results indicate an isolated N atom produces a total magnetic moment of 1.0μB and introduces spin-polarized 2p states in the band gap. The origin of the magnetic moments is the holes in N 2p band of the N dopant. The half-metallic ferromagnetic properties in N–CdO system is mainly driven by N(2p)–Cd(4d)–O(2p)–Cd(4d)–N(2p) coupling chain through the strong p–p interaction between N and O atoms.