Deep eutectic solvents (DESs) have received increasing attention in recent years as designer solvents for bio-processing. In this study, ultrafast microwave-assisted DES pretreatment was developed for lignocellulose fractionation. The pretreatment was conducted for 45 s with microwave irradiation at 800W using a DES composed of choline chloride and lactic acid (ChCl: LA). The results indicated that such pretreatment was highly effective in removing lignin and xylan while retaining most of cellulose in the pretreated solids. The ultrafast fractionation increased the cellulose digestibility by 2-5 folds. Switchgrass gave a similar glucose yield to corn stover but experienced less cellulose loss during the pretreatment. Miscanthus was also substantially fractionated but showed lower digestibility than switchgrass and corn stover. Lignin was readily recoverable with relatively high purity from all the pretreatment liquor. Our study demonstrated an innovative ultrafast pretreatment process with superior effectiveness for biomass fractionation and cellulose digestibility improvement as well as lignin extraction.