Abstract: Two-step acid-base catalyzed silica xerogels with different amounts of additives were prepared through sol-gel and ambient pressure drying. The surface fractal dimensions of these silica xerogels were determined based on a nitrogen adsorption-desorption method by means of the Frenkel-Halsey-Hill (FHH) model. The changes of density, pore structure and specific surface area were compared between pure silica xerogels and the impure ones. And the shape of pores was estimated by the theory of de Boer. The results show that the strength of silica xerogels is increased by the additives, and the shrinkage is restrained in the course of drying. The porous structure and the concentration of alkoxide monomers were also changed. The specific surface area of modified silica xerogels is decreased by adding silica xerogels powder and the pore distribution is widened. On the contrary, the specific surface area is increased by adding TiO₂ powder and short fibers and the maximal value of 1064.96m²/g is achieved. All the surface fractal dimensions of the samples were between 2.4 to 2.5.