A positive active material for a rechargeable lithium battery comprising a manganese oxide-based solid solution is disclosed. The positive active material includes a lithium manganese oxide-based solid solution having a specific surface area of 3 m 2 /g to 12 m 2 /g and a crystallite diameter of 40 nm to 120 nm. A specific surface area and a crystallite diameter falling within said ranges are achieved by controlling, among others, the firing time (specifically in the range 4-24 hours) and the firing temperature (specifically in the range 710-980 °C) of the powder obtained after mixing the co-precipitated hydroxide with a lithium compound. In addition, a positive electrode for rechargeable lithium battery including said positive active material and a rechargeable lithium battery including said positive electrode are also disclosed.