A positive active material for lithium secondary batteries, includes a composite oxide including an oxide which is represented by the composite formula Li.sub.xMn.sub.aNi.sub.bCo.sub.cO.sub.2 and has an .alpha.-NaFeO.sub.2 structure, and an impurity phase including Li.sub.2MnO.sub.3. The values a, b, and c are within such a range that in a ternary phase diagram showing the relationship among these, (a, b, c) is present on the perimeter of or inside the quadrilateral ABCD defined by point A (0.5, 0.5, 0), point B (0.55, 0.45, 0), point C (0.55, 0.15, 0.30), and point D (0.15, 0.15, 0.7) as vertexes, and 0.95