An negative active material for a rechargeable lithium battery includes a nano-composite including a Si phase, a SiO2 phase, and a metal oxide phase of formulation MyO, where M is a metal with an oxidation number x, a free energy of oxygen-bond formation ranging from −900 kJ/mol to −2000 kJ/mol, x, and x·y=2. The negative active material for a rechargeable lithium battery according to the present invention can improve initial capacity, initial efficiency, and cycle-life characteristics by suppressing its initial irreversible reaction.