The electronic structures and optical properties of ZnS and ZnS:Er have been studied using the first-principles plane-wave pseudopotential method.The calculations show that the bandgap of ZnS:Er becomes narrow in the case of Er3+-doping and a new intermediate band between valence band and conduction band is presented.Moreover,with the Er3+ concentration increasing,the insulativity of ZnS:Er presents a decreasing trend,and the absorption spectra exhibit a red-shift,which are in agreement with the previous experimental results.