A new surface deposition method was researched to decrease pore size of ceramic membranes. CaCO3 was chosen to modify the top layer of gamma-alumina membranes prepared by sol-gel processes. Separation of gaseous acetone/water mixtures by vapor permeation was carried out to characterize the membranes. Improvement of membrane separation property after modifications and SEM photographs proved that this new method was effective to reduce the membrane pore size.