This work studies the thermal reaction of Maoming atmospheric residue at a temperature range 400-500 degrees C in sealed tube reactor. It is found that the CS2 insoluble (coke) is formed at 420 degrees C and above and it increases with time and temperature, which can be fitted with a combination of the second-order and autocatalytic kinetics. The loss of heteroatoms in the coke such as sulfur atom accompanied with the condensation of coke. The concentration, g value and linewidth of stable radicals in residue are measured by electron spin resonance (ESR) and discussed to explore the transformation of coke structure. The formation of stable radicals can be expressed by a combination of the two first-order kinetics including CS2 soluble (oil) cracking and coke condensation with activation energies of 168 and 98 kJ/mol, respectively. More than half of stable radicals exist in oil (or in the coke presented in oil) when the coke contents are no > 3.4%, while about 90% of the total stable radicals can be found in the coke when coke contents are > 8.0%. In addition, the variation of g value and linewidth of stable radicals in coke are also studied.