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Switchgrass is a promising feedstock for bioethanol production due to its high biomass yield, potentially low requirements for agricultural inputs and positive environmental impacts. Genetic modification of conventional switchgrass by down-regulating lignin biosynthetic genes provides an option for improving biomass conversion efficiency. In this study, genetically modified switchgrass varieties were pretreated using dilute sulfuric acid at different combinations of acid concentration (0.75, 1, and 1.25 %, w/v) and residence time (5, 10, 20, and 30 min) to further break down the intrinsic complex structure of lignocellulosic biomass, and the yield of total sugar including xylose in prehydrolyzate and glucose in hydrolyzate was used to evaluate modification and pretreatment effectiveness. Preliminary results show that genetic modification caused the increased ratio of acid soluble lignin : acid insoluble lignin, which might be associated with the improved sugar production of some genetically modified switchgrass varieties. Increasing acid concentration or residence time didnt necessarily result in better sugar production due to the increased degradation of monomeric sugars. After pretreatment using 1% sulfuric acid for 20 minutes, the total sugar production of genetically modified switchgrass Al10/9-40 reached 508 mg, which was 22% higher than the maximum yield obtained from the conversional switchgrass.