The density contrast between the subducting slab and the asthenosphere is one of the factors that affects the subduction pattern. Its increase will result in increase of the buoyancy force and accordingly the subduction pattern will change. In this study, the effect of this parameter is simulated by numerical method and for model setup the geological and geodynamic features of Neo-Tethys subduction beneath Eurasia in Eocene have been used. The simulation results indicate that in such a subduction setting, a density contrast of 8 kg/m3 is a critical value and the pattern of subduction in lower and higher than it significantly changes. Also, according to the distributed extension in the Eocene of Iran, it seems that the Neo-Tethys slab was significantly denser than the surrounding asthenosphere (at least 8 kg/m3).