Garnet-like structured metal oxides with the general formula Li(6)ALa(2)Nb(2)O(12) (A = Ca, Sr, Ba) have been prepared by solid-state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides. The powder X-ray diffraction data reveal that Li(6)ALa(2)Nb(2)O(12) compounds are isostructural with the parent compound Li5La3Nb2O12. The cubic lattice parameter was found to increase with increasing size of the alkaline earth ions. The grain size decreases considerably with the substitution of La by the alkaline earth elements under the same preparation conditions. The Ca-substituted compound exhibits both bulk and major grain boundary contributions to the total resistance, while the Sr- and Ba-substituted compounds show mainly bulk resistance with a rather small grain boundary contribution (similar to 14% of the total resistance at 20 degrees C) with further decrease with increasing temperature. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the investigated compounds, the Ba-compound Li(6)aLa(2)M(2)O(12) shows the highest ionic conductivity of 6 x 10(-6) S/cm at room temperature (22 degrees C) and lowest activation energy of 0.44 eV compared with 0.55 and 0.50 eV for the corresponding Ca- and Sr- compounds, respectively. The ionic conductivity is comparable with that of parent Li5La3Nb2O12 and other fast lithium ion conductors known so far.