A numerical study is performed to study the mechanism of heat transfer enhancement for mineral oil in 2:1 rectangular ducts. Three different heating conditions are considered : top wall heated, bottom wall heated, and both top and bottom walls heated. For the three heating conditions, the present numerical results are all in good agreement with the experimental data. For the case of top wall heated, the heat transfer enhancement is caused mainly by the axial velocity distortion due to temperature dependence of viscosity. For the case of bottom wall heated, the axial velocity distortion is the major factor to heat transfer enhancement in the region near the entrance. While near the fully-developed region, the heal transfer enhancement is mainly caused by the buoyancy-induced secondary how. The mechanism of heat transfer enhancement for the case of top and bottom walls heated is more like that for bottom wall heated.