Abstract: Vacuum residue from Saudi Arabia light crude oil (ALVR) was thermally reacted at 395℃ under the atmosphere of nitrogen. Thermal reaction samples were obtained by online sampling device at different reaction times, and the asphaltene aggregate size was determined by small-angle X-ray scattering (SAXS) measurements. The aggregate size of asphaltene decreases from 29~21 nm in the initial stage of coke induction period; during later stage of coke induction period, the asphaltene aggregate size increases rapidly and reaches to 43 nm when thermal reaction time reaches to 150 min. After the stage of coke induction period, the asphaltene aggregate size changes little and is in the range of 46~42 nm. In the thermal reaction process of residue, the aggregate size of asphaltene is jointly affected by the thermal cracking and association-disassociation of asphaltene, but does not show obvious relationship with asphaltene content.