Although nitrogen-doped zinc oxide has been fabricated as a light-emitting diode, the origin of its p-type conductivity remains mysterious. Here, by analyzing the surface reaction pathway of N in ZnO with first-principles density functional theory calculations, we demonstrate that the origin of p-type conductivity of N-doped ZnO can originate from the defect complexes of N(Zn)-V(O) and N(O)-V(Zn). Favored by the Zn-polar growth, the shallow acceptor of N(O)-V(Zn) actually evolves from the double-donor state of N(Zn)-V(O). While N(Zn)-V(O) is metastable, the p-doping mechanism of N(Zn)-V(O)→N(O)-V(Zn) in ZnO will be free from the spontaneous compensation from the intrinsic donors. The results may offer clearer strategies for doping ZnO p-type more efficiently with N.