For a single crystal of B4C, change of the surface composition due to a heat treatment with temperature range from 600 to 1300 °C was studied by Auger electron spectroscopy analysis. The boron concentration of the surface was continuously measured after the heating temperature became constant. The boron concentration decreased with the heating time and became a steady‐state value. The decreasing rate increased as the heating temperature. As the increase of the heating temperature, the boron concentration in the steady state became larger. At 1200 °C, the boron concentration became close to the bulk concentration. Evaporated particles from the surface of B4C were measured by a collector probe to examine the process of the change in boron concentration. It was found that only boron atoms evaporated from the surface. A simple model was applied to explain the change in composition. Estimated activation energies of boron for the diffusion and the evaporation were 4.8 and 2.5 eV, respectively. The present model agreed with the experimental result.