Lithium ion conduction through $Li_{1-X}Al_XTi_{2-X}(PO_4)_3$ system has been studied as a function of $Al^+$ ion substitution and temperature by using ac impedance technique. The AC impedance was measured with Ag blocking electrode at temperatures from -50 to 200℃ in the frequency range from 10 to $10^6$ Hz. The substitution of $Al^+$ ion for $Ti^+$ ion in the $Li_{1-X}Al_XTi_{2-X}(PO_4)_3$ system occurs up to 0.3 mole of $Al^+$ ion, with accompanying a gradual decrease of lattice parameter. The excess substitution of $Al^+$ ion produced a second phase material. It is suggested that the equivalent circuit of this system consists of a series connection of 3 parallel RC (frequency dependent) elements, each of which represents grain interior, grain boundary and electrolyte/electrode interface. The activation energy for $Li^+$ ion conduction through the $Li_{1-X}Al_XTi_{2-X}(PO_4)_3$ system has the minimum value of 0.325 eV at 0.3 mole of $Al^+$ ion substitution. The decrease of activation energy for $Li^+$ ion conduction up to 0.3 mole of $Al^+$ ion substitution is associated with a reduction of $Li^+$ ion conduction tunnel to a suitable size. On the other hand, the increase of activation energy for $Li^+$ ion conduction at excess substitution of $Al^+$ ion presumably results from an ion blocking effect by the second phase. Both the $Al^+$ ion substitution and the addition of a small amount of $Li_2O$ decreased significantly porosity, resulting in an considerable increase in the ion conductivity of the $Li_{1-X}Al_XTi_{2-X}(PO_4)_3$.
