In this paper, we have studied the effect of Er3+ concentration ACEL characteristics of ZnS:ErF3 thin films. It is shown that I(4F9/2→4I15/2) increase with increasing Er3+ concentration. By studying the EL decay, the ratio of green to red emission vs applied voltage and EL efficiencies of 4F9/2 and 2H11/2+ 4S3/2 energy levels of ZnS:ErF3 thin films with different Er3+ concentration, we find that the increase of I(4F9/2→4I15/2) is due to the cross-relaxation processes occurred between Er3+ ions. The cross-relaxation rate increase with increasing Er3+ concentration, and results in the increase of I(4F9/2→4I15/2).