Small angle X-ray scattering (SAXS) with synchrotron radiation as X-ray source h as been used to study the structure of SiO2 xerogels prepared by sol-gel pr ocess. All SAXS profiles in this paper deviate from Porod's law and show negativ e or positive deviation. In order to obtain the information of pore in SiO2 xerogels, we have proposed the corresponding methods to correct the negat ive and positive deviations from Porod's law. Then, the average pore diameter of SiO2 xerogels is determined with Debye's method and Guinier's method , separately, and the results are found to be close to each other. The average d iameters fall in the rangl 3-25nm for samples prepared under various conditions. The results of SAXS are also close to that determined by N2 adsorption met hod at 77K with ASAP2000.