Ambipolar behaviors of zinc oxide nanowires (ZnO NWs) are obtained for the first time by controlled annealing in hydrogen ambient. Optimum annealing can create p-type acceptors into ZnO NWs while maintaining the n-type donors from the as-grown samples. We found that such optimum condition must be one that is more than sufficient to eliminate the green photoluminescence (PL) band of the as-grown ZnO NWs. All of these can be explained by an ab initio model of Zn vacancy-hydrogen complexes, which can served as the donor, acceptor, or green PL quencher depending on the number of hydrogen atoms involved.