The lithium rare earth silicates of nominal composition "LiLnSiO(4)" (Ln = La, Nd, Sm, Eu, Gd, Dy) have been synthesized. The lithium ion conductivity of their sintered samples was found to consist of two contributions; one is the gain boundary composed possibly of lithium silicate amorphous phases and the other the bulk phase with an apatite structure. Although the conductivity of the bulk phase was smaller than that of the grain boundary, it was apparently increased by several orders of magnitude than that of stoichiometric apatites of Li(x)Ln(10-x)Si(6)O(24)O(3-x) with x=2 and x=3. From the structure refinement of the bulk phase, this enhancement in the conductivity of the apatite phase in LiLnSiO(4) was reasonably interpreted by the model where lithium ions and vacancies were both partially introduced to the 4f site of the apatite structure.