In this study, mixed softwoods were pretreated with an ionic liquid, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), and the bioconversion efficiencies to fermentable sugars were estimated through the enzymatic hydrolysis. The cellulose crystallinity, surface morphology, structures and compositions of softwood were significantly changed after the ionic liquid pretreatment was carried out under a wide range of temperatures and reaction times. And, biomass digestibility significantly increased with increasing pretreatment temperature and reaction time. The enzymatic degradation of pretreated softwoods was remarkably improved at the pretreatment of high temperatures via the modification of crystalline cellulose I to a mixture of easily digestable cellulose II and amorphous structure, and partial removal of hemicellulose. The conversion of cellulose to glucose reached more than 90% at relevant conditions and the highest glucose yield was measured to about 78%. Through the study, it was clearly shown that ionic liquid pretreatment is one of the effective methods to produce high fermentable sugars without lignin dissolution from lignocellulosic biomass. (c) 2015 Elsevier Ltd. All rights reserved.