The band structures, density of states and absorption spectra of un-doped and Co-doped ZnO supercells of Zn1-xCoxO (x = 0.0417, 0.0625 and 0.1250) have been investigated using first-principles plate-wave uitrasoft pseudopotential method based on the density functional theory. The calculated results showed that the band gaps are broadened by Co doping in ZnO, which is because of the valence band undergoes a greater shift toward the low-energy region than the conduction band lead to. Moreover, the optical absorption edge exhibits a blue-shift, and the heavier the doping concentration is, the more significant the blue-shift will be, which is in agreement with the experimental results. (C) 2012 Elsevier B.V. All rights reserved.