Defects seriously influence the optical properties and mechanical properties of CVDZnS. The deposition technique was introduced and experiment methods such as SEM, TEM and OM were adopted to study the defects: hexagonal phase, abnormally large grains, micro-cracks, micro-pores and impurities. The microstructures of these defects were observed, and the formation mechanisms of these defects were analyzed. Hexagonal ZnS as an impurity causes anisotropy in different directions with different refraction indexes and increases the scattering of CVDZnS. Increasing the deposition temperature can inhibit the formation of the hexagonal phase. The main reason for the growth of abnormally large grains in CVDZnS is high deposition rate. The lower deposition temperature increases growth stress in CVDZnS, which consequently results in the bowing of CVDZnS plates, and introduces microscopic defects such as grain blending and micro-cracks in the crystal. Micro-pores and impurities are induced by high reactant concentration in the local deposition chamber, which can be avoided by improving the gas flowing so as to make the concentration more uniform.