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The electronic structures of C, C-Al and C-Ga doped ZnO were investigated from the first-principles based on density functional theory (DFT). It was found that the C-doped ZnO where 1/8 Os are substituted by C is p-type with hole carriers locating nearby valence band maximum. Moreover, the Madelung energy increases in this case. Incorporating the reactive donor Al or Ga into C-doped ZnO system at Zn sites, not only enhances the C acceptor concentration, but also gets a shallower C acceptor energy level in the band gap in p-type codoped ZnO crystals and decreases the Madelung energy. (c) 2009 Elsevier B.V. All rights reserved.