Pretreatment of cellulose with acid and hot water at low temperature (200-260 degrees C) was performed to control the pyrolysis products by changing the precursor structure. Acid pretreatment changed cellulose into a partly cross-linked and loose structure, which resulted in a large amount of tar consisting of levoglucosan and high-molecular weight components by flash pyrolysis. On the contrary, hot water treatment caused the suppression of tar formation by imparting a random and highly cross-linked structure to the cellulose precursor. A new parameter index, referred to as the degree of cross-linking for pretreated precursors, X-0, which is based on glucose and oligosaccharide yields produced from acid hydrolysis of pretreated samples, was proposed to analyze the precursor structure. As the degree of cross-linking increased with the severity of the pretreatment conditions, the number of OH groups in the pretreated samples decreased. Through analysis of the solid precursors using this index, a simple structural model was proposed to properly estimate the distribution of the pyrolysis products. The estimated values were in agreement with the product yields experimentally obtained by flash pyrolysis.