The model of the ternary compound ZnCrS2 is constructed by replacing the Zn atoms in the zinc-blend phase ZnS with Cr at two different positions a and b in a ratio of 1∶1. Then the electronic and the magnetic properties of ZnCrS2 are invescigated by using the plane wave pseudopotential (PWPP) method with the density functional theory (DFT). The results show that both kinds of ZnCrS2 are more stable in the ferromagnetic state than in the anti-ferromagnetic state, and are half-metallic ferromagnets with half-metallic band gaps of 0.964eV and 0.755 eV, respectively. The comparison of two different models reveals that ZnCrS2 in a-position replacement is more sable and also has a larger half-metallic band gap. Furthermore, the spin-polarized electronic density of states, the band structure and the magnetic moment of ZnCrS2 are analyzed in detail. The present results should be useful for the future experimental study.