An asphaltene fractionation method was developed in order to investigate the effect of heptane (n-C7) insoluble asphaltene (C7-asphaltene) on residual oil stability. C7-asphaltene was separated into heavy and light fractions by a new method using a binary solvent system of toluene and heptane (n-C7), It was found that the heavy fraction of C7-asphaltene in residual oil, extracted by this method, consisted of highly condensed polynuclear aromatics. Our new fractionation method and the Heithaus stability evaluation method were applied to hydroprocessed residual oils. The peptizability of heavy fraction of C7-asphaltene defined by the Heithaus method decreased in accordance with structural condensation of that fraction. On the other hand, light fraction of C7-asphaltene was considered to influence the peptizing power. We proposed a new conceptual model : light asphaltenes would perform as peptizing material as well as resin, and heavy asphaltene would be peptized in oil. This model introduced from our new asphaltene fractionation method could be more effective for understanding the destabilization phenomenon of residual oil.