Boron carbide (B4C) is used as a neutron absorber for nuclear reactor control and installation protection in most types of reactors. When the B4C is irradiated in the reactor, large quantities of lithium and helium (up to about 10²² /cm³) are produced due to the neutron absorption reaction ¹⁰B (n, α) ⁷Li. This work aims at studying the Li diffusion in large grain size (20–50 µm) or small grain size (0.2 – 5 µm) B4C in the temperature range of 500–800 °C. Lithium was implanted in B4C samples at a fluence of 10¹⁴ ions/cm² (maximum concentration of 54 at. ppm). The Li concentration profiles as a function of depth were obtained before and after each heat treatment by Time-Of-Flight Secondary Ionization Mass Spectrometry technique. A heterogeneous diffusion process was observed depending on the implantation depth. Considering diffusion in the non-damaged, bulk material beyond the implanted zone, the thermal diffusion of lithium into the grain boundaries was found to be about four orders of magnitude higher than into the grains. •TOF-SIMS analyses of Li have been performed in implanted and annealed high density, large grain boron carbide.•The diffusion coefficient of Li in boron carbide has been determined in the 500 – 800 °C range, the obtained activation energy, about 2 eV, is close to ab initio estimations.•Li diffusion is highly enhanced in the gain boundaries.