Alkenyl succinic anhydride (ASA) is well known as one of the synthetic sizing agents widely used in neutral papermaking and its role at higher pH range sizing has become important because of environmental issues, the lifting ratio of recycling pulp, anion trash increase in papermaking system and so on. Although many literatures on ASA have been published, we studied the mechanism of ASA sizing by using both ASA and ASAcid (hydrolyzed ASA) in order to obtain further comprehension. In this experiment. sizing treatment was carried out under ideal conditions where the filter paper was immersed in ASA emulsion or in the toluene solution of ASA, so as to retain the same quantity of sizing material in filter paper. The same procedure was adopted in the case of ASAcid. Contrary to our expectations, the most notable fact from the results obtained is that the sizing effect of the filter paper treated with ASA emulsion is superior to that of ASA toluene solution. These results suggest that the most essential factor to develop sizing is morphology of sizing material distribution on fibers. We speculate that the entirely uniform distribution of sizing material, like a uniform thin film, is not necessary for sizing development but the scattered distribution, like dots, is required.