A negative active material having controlled particle size distribution of silicon nanoparticles in a silicon-based alloy, a lithium battery including the negative active material, and a method of manufacturing the negative active material are disclosed. The negative active material may improve capacity and lifespan characteristics by inhibiting (or reducing) volumetric expansion of the silicon-based alloy. The negative active material may include a silicon-based alloy including: a silicon alloy-based matrix; and silicon nanoparticles distributed in the silicon alloy-based matrix, wherein a particle size distribution of the silicon nanoparticles satisfies D10≧10 nm and D90≦75 nm.