Zn x Mg 1 - x S:Mn thin film electroluminescent (TFEL) devices have been developed in a wide range of compositions, x between 0.16 and 1.0, for use as yellow/green-light-emitting displays. Electroluminescence shows the blue shift alike photoluminescence with the increase in Mg composition. The Zn 0 . 9 8 Mg 0 . 0 2 S:Mn TFEL device driven at 1kHz has a high luminance of 1056cd/m 2 and an efficiency of 1.2lm/W. The CIE color coordinates varied from (x, y)=(0.54, 0.45) at x=0.98 to (0.47, 0.52) at x=0.16 with the decrease in Zn composition x in consistence with the peak shift from 585 to 565nm.