Abstract: Colloidal stability variation of Dagang atmosphere residue (DG-AR) during thermal reaction under nitrogen or hydrogen was comparatively investigated. It was found that the colloidal stability of DG-AR samples decreases sharply along with the thermal reaction until coke is formed, and then the deterioration of the colloid stability is slowed down. At the aspect of the composition and characteristics of residue fractions, it was found that with the proceeding of the thermal reaction, the content of asphaltenes reaches its maximum at the end of coke induction period; the content of saturates and light aromatics fraction increases, while the content of heavy aromatics, light resins, middle resins, or heavy resins decreases. This trend is more prominent for the thermal reaction under nitrogen. The mean dipole moments and mean relative molecular weight of the residue fractions were measured, which suggested that as the thermal reaction is going on, the mean relative molecular weight and the mean dipole moments of maltene fractions decrease, while those of asphaltenes have their maximum values. This variation is consistent to that for the colloidal stability during the thermal reaction. The dipole moment variation of asphaltenes showed that asphaltenes having more polarity tend to be associated and transformed into coke. Furthermore, the catalyst and hydrogen is able to restrain asphaltenes from condensation and coke formation.