Biomass recalcitrance has been recognized as one of the major barriers that hided the cost-effective conversion of lignocellulosic biomass to bioethanol, therefore the current bioconversion process require an essential step known as pretreatment to increase the cellulose accessibility. This thesis provides information about changes in cellulose accessibility upon different pretreatments, along with how these pretreatments alter the chemical and physical structures of biomass, will be extremely helpful to further optimize the current pretreatment process. Multiple promising analytical techniques including Simons’ stain, NMR cryoporometry, relaxometry, mercury porosimetry was introduced and successfully applied on pretreated biomass samples to characterize the cellulose accessible surface area and biomass porosity. Different pretreatments increase cellulose accessibility through different mechanisms to different extent. Dilute acid pretreatment is more effective than steam explosion in terms of increasing accessible surface area of cellulose as reflected by Simons’ stain and NMR cryoporometry, while NMR relaxometry suggested steam explosion is more effective at pore expansion for the cell wall water pools detected by changes in NMR relaxation time. Alkaline pretreatment decreased cellulose degree of polymerization, cellulose crystallinity, lignin content and subsequently increased cellulose accessibility, with sodium hydroxide pretreatment proved to be much more effective compared lime or soaking in ammonia pretreatment. Delignification through alkaline-based pretreatment is found less effective than removal of hemicellulose using acid in terms of cellulose accessibility increase. Lignin didn’t directly dictate cellulose accessibility but rather restricted xylan accessibility which in turn controls the access of cellulase to cellulose. Pore size distribution analysis based on mercury porosimetry also indicated that the most fundamental barrier in terms of biomass porosity scale for efficient enzymatic hydrolysis is the nano-pore space formed between coated microfibrils, despite some of the porous architecture such as cell lumen and pit could be severely destroyed after pretreatment. The action of cellulase on the characteristics of cellulosic fractions obtained from pretreated biomass was also investigated. Cellulose accessibility was found to increase at the beginning of hydrolysis, and after reaching a maximum value then starting to decrease. Enzymatic hydrolysis resulted in a rapid decrease in the cellulose degree of polymerization then gradually leveled off, suggesting the existence of a synergistic action of endo- and exo-glucanases that contribute to the occurrence of a peeling off type mechanism.