zMicrocrystalline cellulose Avicel was subjected to an ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) pretreatment step to obtain a high conversion of soluble sugars in the enzymatic hydrolysis of cellulose. Avicel was dissolved completely in [BMIM]Cl, followed by precipitating it into deionized water to obtain the regenerated cellulose. The high performance liquid chromatography (HPLC) analysis results indicated that after 24 h, the sugar released from IL-treated Avicel was 94.65%, much higher than that from the original Avicel (48.57%). Structure, crystallinity and morphology of the IL-treated Avicel were also investigated as follows: the hydrogen bonding, detected by FTIR, decreased; the crystallinity, monitored by XRD, changed significantly from crystalline to amorphous pattern; the morphology, examined by SEM, demonstrated that the orderly structure was destroyed remarkably with some pores and holes appearing in the cellulose substrates. These changes of Avicel pretreated with [BMIM]Cl led to the increase of sugar conversion during cellulose enzymatic hydrolysis.