A theoretical study of the structural phase transformation of ZnS under high pressure has been performed using first principle plane wave pseudopotential (PW-PP) and full potential linear augmented plane wave method (FPLAPW) calculation in which Zn-3d states are treated as valence states. In both methods, we have used a generalized gradiant approximation for the study of phase transformation and structural parameters. The calculated difference in lattice constants (ฮฮฑ 0 ) by PW-PP and FPLAPW methods for zinc-blende, cinnabar and rocksalt structures is equal to 0.003, 0.01 and 0.001 ร
respectively. There is a very good agreement between the results of PW-PP and FPLAPW calculations that shows soundness of our choice of pseudopotential. The calculated transition pressure for zinc-blende โ rocksalt is in agreement with available measured data. We present calculations of the optical properties for three phases of ZnS. The band gap of different phases of ZnS decreases in order of zinc-blende โ cinnabar โ rocksalt mainly due to red shift of Zn-s states in the lowest conduction band. Besides, the optical band gap decreases from 2.84 eV (direct) to 0.188 eV (indirect). The shift of calculated complex dielectric function ฮต 2 (ฯ) for zinc-blende โ cinnabar โ rocksalt is also discussed in details of optical transition that occurred in different phases.