A large amplitude rattling-type anharmonic vibration of Li is possible without guest-host type structure, as we report here for tetragonal LiNH2 crystal. The low temperature (0.4–300 K) specific heat capacity and Raman spectroscopy support the phonon model of site-specific Li activities governed by the symmetry of the potential energy distribution around the Li atoms in LiNH2, which yields the anharmonic Li3 vibration (optical) in one direction (either X or Y axis of the crystal), while the Li1 and Li2 atoms remain silent. Our finding may help to correlate ionic conductivity, thermal, and hydrogenation properties of LiNH2.