This is the first study to examine the minimum amount of choline acetate (ChOAc), a biocompatible cholinium ionic liquid (IL), for pretreatment of bagasse to achieve the sufficient enzymatic saccharification and co-fermentation from the obtained sugars. Pretreatment of bagasse was conducted using ChOAc at different ionic liquid biomass ratios (IL/biomass ratios of 0-3) to determine the minimum and sufficient IL/biomass ratio for enzymatic saccharification. To apply a small amount of viscous ChOAc to all of the bagasse powder, the ChOAc aqueous solution was first mixed with bagasse powder and the suspension was heated, which pretreated the biomass along with evaporation of water. The minimum IL/biomass ratio was found to be 1.5, which achieved cellulose and hemicellulose saccharification percentages of 95% and 93%, respectively, at low-loading (10 g/L) saccharification. The bagasse pretreated at the IL/biomass ratio of 1.5 was then applied to high-loading saccharification (100 g/L) and subsequent co-fermentation for ethanol production. Glucose and xylose concentrations were 56 g/L and 14 g/L, after 72 h of enzymatic saccharification at high-loading, which corresponded to cellulose and hemicellulose saccharification percentages of 95% and 65%, respectively. In the subsequent co-fermentation from a mixed sugar solution containing 27 g/L glucose and 7 g/L xylose, the ethanol concentration was 15 g/L at 24 h, which was 85% of the theoretical value for the sugar-based ethanol yield.