Zn1-xCoxO nanostructures (x = 0, 0.01, 0.02, and 0.04) were synthesized by a solvothermal process. The samples were characterized by XRD, SEM, TEM, EDS, and UV-vis spectroscopy. The structure and morphology analyses showed that Co doping can slightly promote ZnO crystallinity and influence the nanostructure morphology but cannot change the crystal structures of ZnO samples. The optical measurements showed that Co doping can enhance the visible light absorption abilities of the ZnO samples and that the band-gap energies of the Zn1-xCoxO nanostructures can be effectively tuned from 3.25 to 2.85 eV.