We use a plan-view transmission electron microscope technique to unambiguously image the "physical" interface position between Si and furnace grown SiO2 layers. The effect of postoxidation annealing on the interface roughness of Si/SiO2 was studied with this technique. While no obvious effect due to postoxidation annealing on roughness was observed for silicon (111) a postoxidation annealing at 900 degrees C dramatically removed roughness at Si(100)/SiO2 interfaces. A model was developed to explain our experimental results based on the idea of kinetic smoothening and oxidation induced roughening. Qualitative agreement has been reached.