Amorphous a-LATP (x = 0.3) fine powder with the composition of Li1+x Al x Ti2−x (PO4)3 in Li2O–Al2O3–TiO2–P2O5 system was prepared directly from a mixture of Li2O, γ-Al2O3, TiO2, and P2O5 as starting materials, by using a mechanical milling (MM) technique at room temperature. A NASICON-type Li1+x Al x Ti2−x (PO4)3 (x = 0.3) (c-LATP (x = 0.3)) solid solution was obtained by the heat treatment of mechanochemically prepared amorphous a-LATP (x = 0.3) material. The sintered c-LATP (x = 0.3) pellet showed high lithium ion conductivity of the order of 10−4 S cm−1 at room temperature and activation energy of about 35 kJ mol−1. The high-conducting lithium ion c-LATP (x = 0.3) solid electrolyte fine powders were investigated as surface modification materials of LiCoO2 cathode in lithium cells using nonaqueous electrolyte solutions. The c-LATP (x = 0.3)-modified LiCoO2 electrodes exhibited high capacity (ca. 180 mAh g−1) and good cycle performance with a high charge cut-off potential of 4.5 V (vs. Li/Li+) in lithium cell. The increase in the electrode/electrolyte interfacial resistance for the LiCoO2 electrode modified with c-LATP (x = 0.3) fine powder was inhibited.