Zn1-xCoxO (0≤x≤0.10) nanocrystalline compounds with different compositions were prepared by ball milling, and thin films of these compounds were prepared by electron evaporation method. XRD patterns are used to study the structural properties of these films. All films showed a hexagonal wurtzite structure. Using XRD patterns, calculate crystallite. The optical constants n and k of the Zn1-xCoxO nanocrystalline film are calculated in the range of 300-2500 nm based on K-K method. As Co is more doped, the refractive index also shows an increase. According to the Tauc relationship, the optical energy gap of the Zn1-xCoxO film is calculated, which proves to be a direct transition.