Quantitative saccharification is a basic method to determine sugar composition of lignocellulosic materials. The NREL developed a Laboratory Analytical Procedure (LAP #002) by using a 2-step hydrolysis (72% sulfuric acid, 30C, 2 hours; and 4%, 121C, 1 hour) for converting polymeric carbohydrates to monomeric sugar units, which are measured by HPLC.  In 2006, NREL updated the protocol slightly because some acid-liable carbohydrates degrade greatly during the above processes.  Here we propose a modified QS method to more accurately determining acid liable carbohydrate contents through further decreasing acid-liable sugar degradation. We compared the sugar concentrations of corn stover, switchgrass, wheat straw, Douglas fir, and hybrid poplar by the modified QS method with two existing NREL protocols and found that the xylan concentration was over-estimated by 4.6% by the previous methods.