A positive active material for a rechargeable lithium battery includes a first positive active material including a secondary particle including at least two agglomerated primary particles, where at least one part of the primary particles has a radial arrangement structure, as well as a second positive active material having a monolith structure, wherein the first and second positive active materials may each include nickel-based positive active materials and the surface of the second positive active material is coated with a boron-containing compound. Further embodiments provide a method of preparing the positive active material, and a rechargeable lithium battery including a positive electrode including the positive active material.