In boron carbide, the dependence of EPR line width (DeltaB) with g = 2.0028 on the contents of free carbon, c(C), is used to determine the surface-to-volume ratio (S/V) of free carbon inclusions. Consecutive reduction of cc was conducted by a method of low-vacuum oxidation at a range of temperatures. The amount of a free carbon (a graphite phase) in samples was measured by X-ray. Analysis of changes in DeltaB (c(C)) has shown that free carbon inclusions at the surface layer of polycrystalline boron carbide particles has S/V two times smaller than that of free carbon inclusions in the interior of polycrystalline boron carbide.