Abstract: Liquid N₂ adsorption test was made on lime and cement modified expansi ve soils by automatic adsorption apparatus, and quantitative analysis was made o n micro pores of modified expansive soils through adsorption theory. It was foun d that there exist many slit-like pores inside the modified expansive soils; mi cropo res of expansive soils before and after modification mainly concentrates around 8.4A; mesopores over 30A distribute divergingly; modification materials mainly affect pores over 200A; admixture of modification materials decreases micropore volume of expansive soils, while enhances mesopore volume. Finally, compared with resul ts of adsorption test the scanning electron images of expansive soils were analy zed.