Thermal conductivity of solid silicon as a function of the mole fraction of isotopes was investigated by molecular dynamics simulation. The thermal conductivity of isotope-silicon was calculated by using an empirical potential of Stillinger Weber potential. We employed equilibrium molecular dynamics based on Green-Kubo's formula in which the autocorrelation function of heat flux was integrated as a function of duration time. The results of calculation showed that thermal conductivity of mixed isotope-silicon is smaller than that of pure isotope silicon. The results also showed that a pure isotope with a light mass has a large thermal conductivity. (C) 2004 Elsevier B.V. All rights reserved.