Ferromagnetic semiconductor has attracted much attention for the application of spintronic devices, which will bring next generation of the information technology. Cobalt-doped Zinc Oxides (ZnO: Co) is strong candidate of this material group. The ZnO: Co films were grown on a silicon substrate (100) at room temperature by radio-frequency (rf) sputtering deposition and followed by post-Annealing treatment for 3 hours at 400°C and 800°C in the air. The transition from paramagnetic to ferromagnetic occurs after annealing at 400°C, and the properties return to paramagnetic like as-deposition when the temperature rises to 800°C. The XRD measurement of ZnO: Co films exhibited a wurtzite structure in the (002) plane and was free from secondary phases. Then, post-Annealing at 400°C due to shift peak and decrease oxygen element, meanwhile the crystallinity significantly up to 800°C.