The characteristic emission of Sm + 3 in Zn phosphors, fired in inert atmosphere (N 2 gas) at various temperatures, was not detected. However, fired in reductive atmosphere at 1050 o C, the characteristic Sm + 3 emission appears, which exhibits intra-4f transitions at 570, 606 and 653 nm, as predicted from the 4 G 5 / 2 level to the 6 H 5 / 2 , 6 H 7 / 2 and 6 H 9 / 2 levels. Sm doping favors the formation of hexagonal phase in host lattice of ZnS:Sm. The increase in hexagonal phase content will boost the overall photoluminescence emission intensity. The self-activated luminescence intensity increases with the increase in the amount of Sm doping up to a maximum at 0.2 mol% of Sm dopant. According to the phase identification by X-ray diffraction, the Sm 2 O 2 S phase favors the Sm + 3 characteristic luminescence but it deteriorates in Sm 2 O 3 .