A peptization mechanism of boehmite was proposed in this work. The effects of peptization on the properties and performances of a fluid catalytic cracking (FCC) catalyst were explored. A boehmite colloid was prepared through peptization of boehmite with hydrochloric acid, and its particle size distribution was characterized during peptization. With an increase of the acid/alumina molar ratio from 0 to 0.11, the particle size of the boehmite colloid decreased to 3.5 nm. The particle size increased from 3.5 to 11 nm when the acid/alumina molar ratio was further increased to 0.16 and remained at 10 nm thereafter. The smallest dispersed boehmite particles were obtained in an acid/alumina molar ratio of 0.11. On the basis of many experiments, the dispersion and coalition mechanisms of boehmite during acid peptization were proposed. Boehmite particles absorb protons on the surface hydroxyl groups and repel each other to form colloidal particles when the acid/aluminia ratio is low. With more hydrochloric acid added, large amounts of chloride anions cause compression of the boehmite diffuse layer, thus resulting in boehmite coalition. A FCC catalyst was prepared by peptizing boehmite with an acid/alumina molar ratio of 0.11. The catalyst has a larger external surface area, a higher mesoporous volume, and better acidity distribution than the catalyst prepared with boehmite. Both conversion of residue oil and yields of diesel and gasoline over a peptized catalyst are higher than those over the catalyst without being peptized.