A series of layered perovskite oxides, LiLnTa2O7(Ln˭La, Nd and Sm) was prepared by ion-exchange reaction of RbLnTa2O7powders with molten salt of LINO3. According to Rietveld analysis, it is found that the tantalum ions are remarkably displaced from the center of TaO6octahedra, resulting in a short Ta-Oapexbond due to high covalent bonding character. On replacing the large constituting ion in Ln site with smaller one, TaO6octahedron becomes more distorted, which gives rise to a remarkably shorter Ta-Oapexbond An increasing trend of Ta-Oepexbond strength observed by Raman spectra is well consistent with the reverse trend of Ta-Oapexbond distance from the Rietveld analysis. As the large La ion is replaced by the smaller one (from La to Nd, Sm), the ionic conductivity increases with a decrease of activation energy It is therefore concluded that the strong Ta-Oapexbond leads to a weakening of Li-Oapexbond in view point of bond competition, giving rise to an enhancement of Li ion conduction.