We prepared slurries by dispersing LiMn2O4 and LiCoO2 into toluene or NaPO3 aqueous solution. The slurries were then milled with a high-pressure wet-type jet mill to form fine particles. In estimating the performance of Li cells containing the materials, we found that the cell capacity improved with a decrease in the materials’ particle size. Maximum cell capacity was obtained using materials produced by milling NaPO3 slurry to reduce the particle diameter to 2–3μm. Such the maximum value was disappeared by treating the materials with toluene. The treatment, however, did not provide remarkable increase in the cell capacity. We found that the cell cycle life tended to increase with a decrease in the mean diameter of the material particles.