Heavy oil/bitumen with asphaltene requires high energy consumption for transportation, because of its high viscosity. This often causes serious pipeline problems; therefore, it is necessary to reduce the viscosity to resolve these issues. In this paper, we propose a new simple method to reduce the viscosity of diluted bitumen using partially oxidized asphaltene without removing asphaltene. This partially oxidized asphaltene, modified by exposing asphaltene to ozone, has a strong affinity to nonoxidized asphaltene via pi-pi stacking, hydrogen bonding, and acid-base interaction. The added oxidized asphaltene thus facilitated the agglomeration of asphaltene, so that the hydrophilic part can be hidden, and a small amount (1250 ppm) decreased the viscosity of diluted bitumen by up to 45%. We investigated the change in rheological properties of the diluted bitumen for various oxidized fractions of asphaltene and for various fractions of oxidized asphaltene to asphaltene. We showed that the decreased shear viscosity is likely to originate with the increased particle size of the asphaltene, that is, not the change in interactions between the asphaltene particles, but rather from their nonuniform distribution in size.