A Li2O-Al2O3-TiO2-P2O5 glass was prepared and converted into glass-ceramics through heat-treatment at 700-1000 degrees C for 12 h. X-ray analysis indicated that the resultant glass-ceramics were mainly composed of Li1+xAlxTi2-x(PO4)(3) phases. The lithium ion conductivity of the glass-ceramics increased with the growth of the Li1+xAlxTi2-x(PO4)(3) phase and a extremely high value, 1.3 x 10(-3) S cm(-1), was obtained at room temperature. This value is as large as that of Li3N which is well known to exhibit the highest lithium ion conductivity for a solid electrolyte.