In order to improve the ionic conductivity of LiGe2(PO4)(3) as a solid electrolyte for lithium batteries, we have examined the effects of Al3+ and Y3+ substitution for Ge4+ and of LiOH . H2O addition on the ionic conductivity, The ionic conductivity of LiGe2(PO4)(3) is enhanced four orders of magnitude by Al3+ addition, i.e. 1.3 X 10(-4) S cm(-1) at 23 degrees C in case of the Li1.4Al0.4Ge1.6(PO4)(3). The addition of Y2O3 or lithium salt also enhances the ionic conductivity because of the acceleration of the sintering process by the second phase of Li4P2O7.