To accurately determine the thickness of ultrathin SiO2 films, the influence of roughness on ellipsometric measurements must be examined. Although ellipsometry has been applied to study roughness, quantitative relationship of Si surface roughness measured by ellipsometry and atomic force microscopy is not available. This leads to difficulties in estimating the influence of roughness on oxide thickness measurements. Here we first establish the correlation of roughness measured by these two techniques. Based on such a relationship, we can explain the discrepancy of oxide thickness measured by ellipsometry and surface sensitive techniques.