Provided is a negative active material and a lithium secondary battery including the negative active material. The negative active material for a secondary battery includes silicon particles, wherein circularities of the particles are determined by equation 1 below, and the circularities are 0.5 or greater and 0.9 or less, Circularity=2(pi×A)1/2/P  [Equation 1] where A denotes a projected area of the silicon particle that is two-dimensionally projected, and P denotes a circumferential length of the silicon particle that is two-dimensionally projected.