Hydrolysis of crystalline cellulose is a crucial step in utilization of cellulosic biomass and is generally the bottleneck in the biochemical process. The combination of pre-treatment and the use of suitable enzymes is the key to effective saccharification, and ammonia pretreatment is a promising technique to enhance the velocity and yield in saccharification of crystalline cellulose. In the present study, we heterologously expressed recombinant cellobiohydrolase II from Phanerochaete chrysosporium ( Pc Cel6A) in Pichia pastoris . We then employed surface density analysis to compare the velocities of degradation of crystalline cellulose III_I, which was prepared from algal cellulose I by supercritical ammonia treatment, by the recombinant enzyme and Trichoderma reesei cellobiohydrolase I ( Tr Cel7A). The hydrolytic velocity of crystalline cellulose III_I by Pc Cel6A was approximately 4 times faster than that by Tr Cel7A, though velocity of cellulose I degradation by Pc Cel6A was almost half of that by Tr Cel7A. Since adsorption of both enzymes on cellulose III_I is no more than twice that on cellulose I, we speculate that the enhanced hydrolysis of cellulose III_I by Pc Cel6A than Tr Cel7A is not simply due to the increased surface area, but also reflects higher accessibility of cellulose III_I to Pc Cel6A.