The sintering behavior and electrochemical properties of garnet-like Li(6.25)M(0.25)sLa(3)Zr(2)O(12) (M: Al3+ and Ga3+) were investigated. The sintering of Li6.25Al0.25La3Zr2O12 progressed at 1050 degrees C. On the other hand, the densification of Li6.25Ga0.25La3Zr2O12 pellets progressed at lower temperatures above 950 degrees C to yield relative densities of 93.9% after sintering at 1000 degrees C. The temperatures at which the sintering of Li6.25M0.25La3Zr2O12 progressed are in good agreement with the temperatures where the liquid phases of Li-M-O compounds form. The sintering of these compounds was consistent with the change in the lattice parameter due to the replacement of Li+ for Al3+/Ga3+ in the structure. These results suggest that the cation diffusion between the grains and the grain boundaries due to the replacement of Li+ for Al3+/Ga3+ in the garnet-like structure improves the grain growth of these compounds. Dense pellets of Li6.25Ga0.25La3Zr2O12 were obtained by sintering at 1000 degrees C for 12 h without the use of mother powders, and the bulk and total conductivities obtained at 25 degrees C were 1.6 x 10(-3) and 9.6 x 10(-4) S cm(-1), respectively. The activation energies for the bulk and total conductivities were 0.25 and 0.32 eV, respectively. Li(6.25)Gn(0.25)La(3)Zr(2)O(12) was stable in contact with lithium metal, even for a long period of 1 month.