Abstract: ZnS has high transmittance in waveband 3~5 μm and 7.7~9.3 μm despite its inferior hardness and wear-resisting, which performance can be improved by coating a diamond like-carbon on its surface. It's so hard to coat diamond likecarbon on ZnS directly that the method of films of matching and transition is selected. This paper prepared films of transition and matching that adhere to both the diamond like-carbon and ZnS. The diamond like-carbon film is coated on the matching film by RF-PECVD successfully and the mechanical property affected by the RF power and gas pressure is researched. The results show that the transmittance of the ZnS is more than 90% both on the waveband 3~5 μm and 7.7~9.3 μm, and the hardness is nearly 5 times as the ZnS. Its optical and mechanical property has no change after the environment test.