Reaction of glucose and xylose to secondary hydrolysis of concentrated acid hydrolysis was quantitatively analyzed by $^1H$-NMR spectroscopic method. Anomeric hydrogen, furan and formic acid peaks were selected for quantitative analysis. The glucose was converted to the formic acid and the levulinic acid via the 5-hydroxymethylfurfural (HMF) but the xylose was converted to the fufural, which further degraded to the formic acid. The conversion to furans was slower for the glucose than the xylose. But the 5-HMF formed from the glucose was unstable in acidic aqueous medium, resulted in fast conversion to the levulinic acid and the formic acid. The furfural was relatively stable than 5-HMF at acidic aqueous medium.