The average thickness of the interface layer wrapped about sols usually is determined by fitting the Porod curve that shows a negative deviation from Porod's law.In this paper we show that it could also be determined by a new method that includes the following steps:(1)determining the average radius R1 of the sol particles including interface layer from the small angle X-ray scattering data in which shows negative deviation from Porod's law;(2) determining the average radius R2 of the sol particles not including the interface layer from the scattering data in which has been corrected the negative deviation from Porod's law;(3) the difference ΔR between R1 and R2,i.e. ΔR=R1-R2,is just the average thickness of the interface layer wrapped about sols.By using the above method,the average thickness of the interface layer wrapped about SiO2 sols prepared under different conditions were determined.