Gasification technology as a feasible clean and efficient utilization technology has been developed to convert the low-grade energy resources like biomass into high value syngas.The generation of syngas is often accompanied by tar formation, which causes blockage in the downstream processing as tar condenses. The elimination of naphthalene and pyrene as heavy tar model compounds by steam reforming was studied by experiments in a horizontal tube reactor and simulation with CHEMKIN. A good agreement between the measured and calculated results of light gas products and soot was obsvered, and one can found that the reactivity of naphthalene is higher than that of pyrene in the presnece of steam and most of carbon of both hydrocarbons convert into soot instead of light gaseous products. The reaction pathways of steam reforming were developed by sensitivity analysis and rate of production. Benzene and naphthalene, represented as precursor of light gas product, are dominated intermediate components of naphthalene and pyrene. The simplified reaction schemes including the reaction pathway as well as the associated kinetics were derived by CHEMKIN.