Abstract: An investigation was made regarding the variation in internal friction and relative modulus with Li₅La₃Ta₂O₁₂ compound by means of force vibration. Two apparent relaxation internal friction peaks were observed. By fitting the data with two Debye peaks, the activation energy and preexponential factor of the peaks are deduced as 0.8-1.1 eV and 10-14-10-26 s, respectively, which indicates the occurrence of mutual interaction between the relaxation species (lithium vacancies). By fitting the data with the coupling model, the activation energy is deduced as 0.6-0.7 eV, close to the activation energy of lithium ion conductivity. Considering the crystal structure of Li₅La₃Ta₂O₁₂ compound, these two peaks are associated with the lithium ion diffusion between adjacent 48g24d sites and 48g48g sites, respectively. The peaks gradually shift toward higher temperature and increase in both height and activation energy when the sample is aged at room temperature in air. Combined with the change of sample’s mass, XRD and TGA, it can be concluded that the protons from the moisture substitute the lithium ions initially occupied the 48g sites in Li5La3Ta2O12 compound to form Li₂O and new protonic derivatives, Li₅-xLa₃Ta₂O₁₂-x(OH)x (0<x≤2.15).