Zinc Sulfid with chemical formula ZnS is one of the first discovered semiconductors with a wide energy band gap and many applications. It is observed that sulfur deficiency affects the optical properties of this crystal. Therefore, in this project we employ density functional calculations to investigate the effect of sulfur deficiency on the structural and electronic properties of ZnS. In the first step, we investigate the effect of this point defect on the structure of ZnS. For this purpose, different concentrations and configurations are considered for the defective crystal, and by computing and comparing the minimum energy of these configurations, the most stable configuration for each concentration is identified. Then, we calculate and compare the electronic structure of the most stable configuration of defective crystals with electron structure of the perfect crystal and analyze the effect of sulfur deficiency at different concentrations on the electronic properties of the system. A new scheme, called DFT-1.5, is used to overcome the conventional DFT error in the band gap calculation of semiconductors and obtain accurate electronic structures.