The quenching mechanism of luminescence by heavy Sm doping in ZnS was investigated by the Rutherford backscattering ion channeling (RBS/C) method. Red luminescence around 650 nm due to the 4f intra transition of Sm3+ ion was observed in the Sm-doped ZnS, and the emission intensity becomes maximum at an Sm concentration of about 2 x 10(19) cm(-3). The RBS/C measurements showed a much shallower dip in the angular profile of Sm yields and a shallower and broader Zn-dip in the heavily Sm-doped ZnS than those in the moderately Sm-doped sample. These results indicate that the quenching of the luminescence by heavy doping of Sm is caused by the deterioration of the structural quality of ZnS and the large displacement of Sm atoms from the lattice sites.