Three lignin fractions with different molecular size (Fraction 3 > Fraction 2 > Fraction 1) were prepared by sequential solvent fractionation of soda lignin. These were structurally investigated by several analytical techniques, such as GPC, P-31-NMR, 2D-HSQC-NMR, and TGA. They revealed that F1 was the smallest and the most thermally labile lignin fraction. Pyrolyzing lignin fractions at 600 degrees C with zeolite Y produced aromatic hydrocarbons (benzene, toluene, xylenes, and naphthalenes) with several types of monomeric phenols. Total pyrolysis product yields were the highest for F1, followed by F2 and F3. Additionally, aromatic hydrocarbons formation was inversely proportional to molecular size and abundantly produced with F1 (35.0 mg/g). They gradually increased in all fractions as the pyrolysis temperature increased (up to 800 degrees C). The transformation behaviors of functional groups were also investigated by using lignin model compounds. Lignin fractions prepared in this study had not only molecular size but also different phenolic hydroxyl (Phe-OH) content which would play a deleterious role in aromatic hydrocarbon formation during zeolite-catalyzed pyrolysis. Therefore, we prepared methylated lignin from each fraction (Methylated fraction 1, 2, and 3) and these were also pyrolyzed under same condition to investigate the independent effect of molecular size.