We have investigated the magnetic properties of Cu-doped ZnO both in bulk and nanocluster by the first-principles calculations.It is found that the magnetic phase has a transition from FM phase to AFM phase as the the size for the doping system decreases from bulk to nanocluster.The Cu ions have a tendency to assemble.In Cu-doped bulk ZnO,the ferromagnetism arises from the strong hybridization between Cu 3d and O 2p orbitals.