Although dilute magnetic semiconductors have promising potentials in spintronics applications,the mechanism of their ferromagnetism remains ambiguous.The extensive theoretical models and exotic experimental evidences provide self-consistent but usually contradictory explanations on its either intrinsic or extrinsic origins.We find room temperature ferromagnetism in a series of Zn1_xCoxO (0.03 ≤ x ≤ 0.10) thin films prepared using magnetron co-sputtering method and treated with post-annealing at temperatures 350 ℃ and 500 ℃.The origin of the ferromagnetism is investigated in terms of electronic structure combining hard x-ray absorption spectroscopy (XAS) at Zn and Co K-edge,and soft x-ray XAS at O K-edge and Co L2,3-edge.The full multiple scattering theory is employed to reinforce the interpretation of the XAS spectra,which concludes that full substitution of zinc by cobalt is responsible for the room temperature ferromagnetism due to the d states of cobalt within the framework of bound magnetic polaron.Moreover,the evidence of cobalt nanoclusters is detected at highly doped and annealed samples.The first principles calculation confirms the electronic structural evidences via the formation energy.