ZnS buffer layers were used to grow ZnO films on GaAs(001) substrates. The role of ZnS buffer layers in the growth of zincblende ZnO films on GaAs(001) substrates was investigated by atomic force microscopy. X-ray diffraction, and photoluminescence (PL) measurements. The optimization of the ZnS buffer layer thickness resulted in improvements of the surface morphology and crystalline quality of ZnO films by homogeneous nucleation. With the optimized ZnS buffer layer thickness of similar to 72 nm the surface root-mean-square roughness of the grown ZnO film was minimized to similar to 14 nm and the deep-level PL intensity was reduced to 1/76 of the near-band edge PL intensity at room temperature.