The lithium ion conductivity in the perovskite-type xLiTaO(3)-(1-x)SrTiO3 solid solution (0.30 less than or equal to x less than or equal to 0.50) has been investigated. The highest ionic conductivity of 5.5X10(-4) S cm(-1) at 300 K appears at x=0.50. The ion conductivity decreases with a decrease in x and then rapidly decreases in the vicinity of x=0.30. This threshold for the ion conductivity is the same as the site percolation threshold for a simple cubic lattice. This indicates that the lithium ion conduction occurs three-dimensionally among the A-sites of the perovskite.