Carbon-doped ZnO films were prepared on c-cut sapphire substrates by conventional PLD method. All films exhibit RT ferromagnetism. However, the samples deposited at low temperature and low pressure with low C concentration show large M _s. The substitution of carbon for O in ZnO causes the electron transfer of 3d orbital of Zn ion, which results in the generation of a net spin with one half in the d-orbit of Zn, and accordingly makes ferromagnetism possible. XPS and Auger results indicate the formation of donor defects such as V_O or/and Zn_i in the film, which makes the net spin align, accordingly induces magnetic ordering of the films. Both a certain number of donor defects and the net spin of Zn ions caused by the substitution of O by C are two key factors in inducing magnetic ordering in C-doped ZnO films. Therefore, defect-induced ferromagnetism is reasonable to explain the observed magnetism in our experiment.