The electronic structures of ZnO and (Zn,Al)O are investigated by using the first-principles pseudopotential plane wave method in the generalized gradient approximation. The effects of Al doping on the bonding of ZnO and the interaction between electrons are analyzed from atomic population, bond population, energy band and electronic density of states based on the molecular orbital theory. Carrier concentration of (Zn,Al)O is calculated from the first-principles calculations, furthermore the change in ZnO conductivity is analyzed. The carrier concentration and the conductivity of ZnO are increased significantly by Al doped ZnO compared with the experimental results.