To improve the performance of the negative electrodes for lithium-ion batteries, a study has been made of the particle-size effect of carbon powder on discharge capacity and the optimum mixing ratio between large (average diameter: 25.8 mu m) and small (average diameter: 4.2 mu m) powder particle size has been found. The largest capacity is obtained when the large particle size fraction is about 70 wt.%. The smaller the particle size ratio, the larger the capacity, where the particle size ratio is the ratio of the average diameter of the smaller component to that of the larger one. These results indicate that discharge capacity is closely related to carbon powder packing, which is controlled by the mixing weight and particle-size ratios.