All Zn1−x Er x O (x=0.04, 0.05, and 0.17) films deposited on glass substrates by radio-frequency reactive magnetron sputtering exhibit the mixture of ferromagnetic and paramagnetic phases at room temperature. The estimated magnetic moment per Er ion decreases with the increase of Er concentration. The temperature dependence of the magnetization indicates that there is no intermetallic ErZn buried in the films. The ferromagnetism is attributed to the Er ions substitution for Zn2+ in ZnO lattices, and it can be interpreted by the bound-magnetic-polaron model.