Abstract: The thermal reaction of Karamay residue oil with three kinds of fine carbonous particle additives was carried out in a batch reactor under N2 atmosphere to investigate the effect of additives on coke formation. The initial N2 pressure was 2.0MPa, the reaction temperature was 420℃ and the additive content was 1w%. The results show that the fine carbonous particles restrain coke formation during thermal cracking of residue oil. The coke restrainability of additives related to their surface properties directly. The additive easily wetted by polar component absorbs asphaltene from asphaltene toluene solution easily, and it also has the stronger ability of restraining coke formation. The abilities of being wetted by polar component, asphaltene absorption and restraining coke among three carbonous additives have the same order: carbon black 1 > active carbon powder > carbon black 2. However, the three abilities have no direct relationship with their specific surface areas. Reactions at different time show that the additives could only restrain the coke formation at the initial period. Once the coke yield is grate enough, the additives does not work. The scanning electronic microscopy (SEM) and thermogravimetric (TG) analysis show that the TI with carbonous additive has less and smaller toluene insoluble spheres than TI without additive. It is concluded that the wetting and spreading of asphaltene and coke precursor phase on carbonous additives are the main reason of restraining coke formation. They constrict the coalescence and growth of coke precursor and coke, and thus reduce the coke yield at a certain reaction time.