Defect chemistry at high temperatures in layered lithium transition-metal oxides of LiCoO2, LiNiO2, and Li(Li1/3Mn2/3)O2 is investigated on the basis of first-principles calculations. The antisite transition-metal ions are the major defects in LiCoO2 and LiNiO2. However, the easy formation of the electron defect in LiNiO2 leads to the preferential valence state of NiLi 0 and thus to the P O 2 − 1 / 2 dependence of the defect concentration on the oxygen partial pressure. On the other hand, the formation of the electron defect as the accompaniment of the antisite cobalt ion in LiCoO2 leads to the preferential valence state of CoLi + and the P O 2 − 1 / 4 dependence. The defect concentration is, therefore, more sensitive to the synthesis conditions for LiNiO2 than that for LiCoO2. Li(Li1/3Mn2/3)O2 with low defect concentrations can be easily synthesized at ambient oxygen partial pressures, although the concentration of the oxygen vacancy increases as oxygen partial presure decreases. The defect chemistry based on the first-principles calculations can provide quantitative information on the characteristics of electrode active materials as well as guides to their optimum synthesis conditions.