The electronic structure and magnetic properties of Co-doped ZnS (zinc blende) were studied with the first-principle method using PW91 within the generalized gradient approximation (GGA) for exchange-correlation. Geometric parameters, formation energies, magnetic moments, density of states, and electron charge densities were investigated. It is found that the formation energy is less than 0 as one Co atom is doped into the middle of the super cell (2×2×2), which indicates that this structure is stable for Co-doping. And the density of states (DOS) near to the Fermi leVel (EF) indicates the presence of a half-metallic character which is caused by p-d hybridization between the Co 3d and S 3p. When two Co atoms are doped into the ZnS (zinc blende), antiferromagnetic (AFM) state is the most stable configuration in the system of Co-S-Co. But in the case of Co-S-Zn-S-Co, the most stable configuration is ferromagnetic (FM) state