To study the relationship between the electrical conductivity and the coking performance, a device for the online measurement of the conductivity of heavy oil during thermal reaction was constructed. Three samples with different fraction compositions, including Middle East atmospheric residue (ME-AR), Tahe atmospheric residue (TH-AR), and Saudi Arabia vacuum residue (SA-VR), were selected for study. The conductivity of all samples presents the peak value, before or after the end of the coke induction period. The molar conductivity of asphaltenes, resins, or aromatics in toluene solutions was measured, and the fractions were derived from SA-VR thermal reaction samples under hydrogen. The molar conductivity of asphaltenes in toluene was 20 and 2000 times more than that of resins and aromatics, indicating that the asphaltene was the most important conductive fraction in the petroleum residue. The dipole moments of fractions of SA-VR thermal reaction samples were measured. The concentration of asphaltenes went through a maximum during a reaction, which was consistent with the conductivity variation qualitatively. This work indicated that there were certain relationships between the conductivity variation and the coking characteristics of residue during thermal reaction.