Disclosed is a negative active material having a reduced volume change during charge/discharge. A fine crystalline region exists on a matrix of an alloy, and lithium is easily dispersed due to the fine crystalline region. In the negative active material, a ratio of the fine crystalline region is represented by an amorphization degree, and is optimized in designing a battery through a measurement of an expansion rate after 50 cycles. The negative active material is used for a secondary battery and includes an alloy formed by a chemical formula below: (a ratio of Ti to Fe is 1:1 and a ratio of Si:Ti or Si:Fe has a range of 5:1 to 9:1.) Si—−xTiyFezAlu where x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09 to 0.14, z: 0.09 to 0.14, and u: larger than 0.01 and less than 0.2.