The susceptibility of cellulose to enzymatic hydrolysis is affected by the structural features of cellulosic materials. It has been suggested that the crystallinity and surface area of cellulose fibers are the most important structural features in this regard. This study investigated in depth the relative effects of these two structural features upon the enzymatic hydrolysis of cellulose and the change of the structural parameters of cellulose during the course of hydrolysis. It was found that the hydrolysis rate is mainly dependent upon the fine structural order of cellulose which can best be represented by the crystallinity rather than the simple surface area. Monitoring the changes in the structural parameters during the course of reaction showed that surface area is not a major limiting factor that slows hydrolysis in its late stages as has been suggested. This information concerning structural features is used to elucidate the mode of action of cellulase.