Thermal diffusivities for the isotope-enriched samples, which are B-10(4) C-12, B-10(4) C-13, B-11(4) C-12 and B-11(4) C-13, were measured by laser Aash method over the temperature range from 290 to 1770 K. Thermal conductivity decreased with the molar weight of boron carbide, The heat transfer in boron carbide is considered to be mainly caused by acoustic phonons. As heavier isotopes were substituted for lighter ones, the phonon velocity in B4C decreased and so did the thermal conductivity. Consequently, total heat transfer by phonons is thought to decrease with molar weight.