Amorphous fine powder in the Li2O-Al2O3-TiO2-P2O5 (LATP) system were prepared directly from a mixture of Li2O, Al2O3, TiO2, and P2O5 as starting materials using a mechanical milling technique at room temperature. LATP glass-ceramics were obtained by heat treatment of the mechanochemically prepared amorphous powder over the crystallization temperature. X-ray diffraction peaks due to LiTi2(PO4)3 and AlPO4 crystals for LATP glass-ceramics were observed. Thus, high lithium ion conducting LATP glass-ceramics solid electrolytes were prepared successfully from the mechanochemically prepared amorphous powder. The LATP glass-ceramic fine particles were investigated as coating materials of LiCoO2 cathode in lithium ion batteries. The LATP glass-ceramics coated LiCoO2 electrode materials exhibited a good charge-discharge performance for charging up to a high voltage (4.5 V vs. Li/Li+).