Disclosed is a method of preparing a positive active material for a rechargeable lithium battery including adding first and second compounds to a solvent to prepare an acidic solution with a pH from 0.01 to 3, the first compound including an element that is capable of forming a double bond with an oxygen of a lithium metal oxide, and the second compound including at least one element selected from the group consisting of alkali metals, alkali earth metals, group 13 elements, group 14 elements, transition metals and rare-earth elements; adding a lithium-containing compound to the acidic coating solution to coat the lithium-containing compound; and heat-treating the coated lithium-containing compound to form a surface-treatment layer comprising a compound represented by the formula MXO.sub.k where M is an element selected from the group consisting of alkali metals, alkali earth metals, group 13 elements, group 14 elements, transition metals, rare-earth elements and combinations thereof; X is an element that is capable of forming a double bond with oxygen; and k is an integer from 2 to 4.