Co-doped ZnO polycrystalline was synthesized with sol-gel method. Room temperature ferromagnetism （FM） was observed in the synthesized sample. Studies onits structural and optical properties indicate that the FM is originated from the Co substitution for Zn in ZnO crystalline lattice, and the temperature of sample synthesis significantly affects the FM and photoluminescence of the corresponding samples, suggesting that Zn interstitial could be responsible for the origin of the FM.