The changes in structures by thermal pretreatment of heavy alkylaromatics, which are very difficult to nuclear hydrogenate, and the effects of the pretreatment on subsequent nuclear hydrogenation were investigated in comparison with catalytic pretreatment of the same feedstock. The following results were obtained : (1) the structures of the thermally pretreated products are quite different from those of the catalytically pretreated ones; (2) subsequent nuclear hydrogenation of the thermally pretreated products becomes much easier because the steric hindrance is removed; (3) in the case where the same feedstock was catalytically pretreated, the products were difficult to nuclear hydrogenate. These results suggest various applications, including heavy oil hydrotreating.