To trace the behavior of naphthalene in the pyrolysis of coal tar, the pyrolysis of coal tar containing C-14-labeled naphthalene was carried out at 800-950 degrees C for 40-50 s. It was found that naphthalene has significant pyrolysis reactivity. The distributions of the radioactivities in compounds with 3-5 rings were 21.6, 30.3, and 18.6% at 800, 900, and 950 degrees C for 50 s, respectively. Furthermore, the distributions of the recovered radioactivity in THFI at 800, 900, and 950 degrees C for 50 s were 1.6, 5.3, and 47%, respectively, indicating that about half of the C-14-naphthalene can be converted into tetrahydrofuran insoluble (THFI) in 50 s at 950 degrees C. This C-14 tracer method using C-14-labeled naphthalene is convenient for tracing the reactivity of coal tar and for elucidating the reaction mechanism of coal tar pyrolysis.