A lithium composite oxide (LiaNi(1-x-y)CoxMyO2, where M is at least one metal atom selected from the group consisting of Al, Ca, Mg and B, anull0.97null1.05, xnull0.1null0.3, and ynull0null0.05), prepared by a method including the steps of: (a) coprecipitating a NinullCo composite hydroxide by adding an aqueous ammonia solution as a complexing agent, and an alkaline solution as a pH-adjusting agent, to an aqueous mixed solution containing a cobalt salt and a nickel salt; (b) adding lithium hydroxide to the composite hydroxide and thermally treating the mixture at 280null420null C.; and (c) thermally treating the resultant of the step (b) at 650null750null C. The average particle diameter of the lithium composite oxide decreases, or the tap density thereof increases, depending on the coprecipitation time. When the lithium composite oxide is used as a positive electrode active material, a lithium ion secondary cell having a high capacity can be obtained.