In order to investigate the effect of Mg, Al doping on the electronic structure of LiCoO2, and to find the mechanism of high electronic conductivity in Mg-doped LiCoO2, we have carried out first-principles calculations based on density-functional theory (DFT) for LiCoO2, Li(Co, Al)O2 and Li(Co, Mg)O2 systems. Studies of band structures and densities of states show that hole states, which enhance the conductivity of semiconductor, appear in the valence bands of Mg-doped material, and the effect of charge disproportionation changes the distribution of Co-3d states in different energy levels. The ionicity of O2- increases with Mg and Al doping.