Structural and magnetic properties of (ZnO)₁₂ clusters doped with one (monodoped) and two (bidoped) Ni atoms were studied with a first-principles method. Substitutional, exohedral, and endohedral dopings are considered. Exohedral isomers are found the most favorable for both monodoped and bidoped clusters. Magnetic coupling between Ni atoms at the nearest neighbor position is mainly governed by competition between direct Ni-Ni antiferromagnetic interaction and ferromagnetic interaction between two Ni atoms via O atom due to strong p-d hybridization. Most importantly, exohedral and endohedral bidoped clusters favor ferromagnetic state, which has potential applications in nanoscale quantum devices.