A series of n-type Cu-doped ZnO thin films was prepared by magnetron sputtering. Such films have shown ferromagnetic properties at room temperature. The Cu ion is in a univalent state as identified by x-ray photoelectron spectroscopy. The moment per Cu ion decreases as the Cu concentration increases. The largest moment about 0.52 μB/Cu was observed for Zn0.95Cu0.05O thin film. The Curie temperature about 360 K was observed for Zn0.95Cu0.05O:N film. To explore the relationship between ferromagnetism and carrier density, nitrogen was introduced into the samples which resulted magnetization and the transition temperature decreasing. These results indicate that the electron density plays an important role in ferromagnetism.