We have performed the ab initio calculations based on density functional theory to investigate the B3-B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C-11, C-12, C-44, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (131). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy-pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for 131 -phase and 20 GPa for B3-phase. (C) 2008 Elsevier B.V. All rights reserved.