ZnS is one of the excellent materials that are used as long wave window in infrared, but the reflection of the surface is high. In order to reduce the reflection of the ZnS surface, we designed the sub-wavelength antireflective structure on ZnS by coupled-wave theory. The parameters of the sub-wavelength antireflective structure were optimiged and the two-dimensional sub-wavelength antireflective structure on ZnS was fabricated by reactive ion etching technique. The results show that the transmittance of etched ZnS is significantly better than that of bare ZnS at wavelengths of 8—12 μm. This is a new antireflection method using ZnS.