We report on a method to quantitatively measure the local energy distribution of surface states density within the band gap of semiconductors using Kelvin probe force microscopy. The method is based on scanning a cross-sectional pn junction; as the tip scans the junction, the surface states position relative to the Fermi level changes, thereby changing the surface potential. The energy distribution is then obtained by fitting the measured surface potential. The method is applied to an oxidized Si (110) surface where a quantitative states distribution across most of the bandgap is obtained.