Inelastic neutron scattering has been used to measure the phonon dispersion relation along (001), (110) and (111) directions of a single crystal of Na2S at 50K. A simple shell model has been successfully fitted to the data. Dynamic quantities such as density of states, Debye temperature, displacement-displacement correlation functions and mean square ionic displacement are computed from the model and compared with CaF2. Finally some remarks on the ionic conductivity of antifluorites are given.