A systematic study of the effect of microstructure on the EL properties of ZnS:Er thin film devices fabricated by thermal evaporation was carried out using XRD, XPS and EL techniques. The experimental results indicate that the high brightness of the devices is attributed to the deposition growth of crystallites oriented in the (311), (400) directions. This can be explained by assuming a higher population of erbium being in the trivalent charge state in the (311), (400) planes of the films grown under certain deposition condition. It is concluded that only the part of rare earth dopant being in trivalent charge state in the film contributes to luminescence. The mechanism of the formation of luminescence center and the excitation of Er3+ion by the electric field are discussed.