A process was proposed for upgrading heavy oil using non-thermal plasma technology in a conventional thermal cracking system under atmospheric pressure. Results from a comparison of the reactivity of a N-2, H-2 and CH4 plasma showed that the plasma can increase the trap oil yield significantly. The trap oil yield increased by similar to 9% when the N-2 plasma was applied and showed a further increase of similar to 19% when the H-2 or CH4 plasma was applied. A detailed study on the H-2 plasma-enhanced upgrading process was carried out and the results showed that the trap oil yields of the plasma-on runs can be 8-33% higher than those of the plasma-off runs, depending on experimental conditions. Compared with the plasma-off runs, trap oil from the plasma-on runs had a higher (H/C)(atomic) but less heteroatoms (S and N). Over-balanced hydrogen in the products from plasma-on runs revealed the H-2 plasma reactivity, which was further demonstrated by an increase in the substitution and condensation indices of trap oil from the plasma-on runs. Although thermal cracking was mainly involved whether the plasma was applied or not, the electrical field for generating the plasma and the generated plasma may assist with hydrocarbon bond cleavage. This was shown by the increased trap oil yield with the N-2 plasma and the hydrogen and carbon residue distribution. Compared with the feedstock, more aromatic and gamma-hydrogen (H-A and H-gamma, respectively) and less alpha- and beta-hydrogen (H-alpha and H-beta, respectively) were present in the residues, which agrees with the bond dissociation energy data. Similarly, the amounts of saturated (C-s) and alkyl (C-p) carbons in the residues were significantly lower than those in the feedstock while the amount of aromatic carbons (C-a) in the residues was higher than the feedstock. The changes in hydrogen and carbon distribution were more significant for the plasma-on runs. This implies that mainly side chain losses and bridged bond breakage are involved in the processes. This was demonstrated further by the molecular weight distribution. In general, the molecular weight of the residues was lower than that of the feedstock, especially for residues from the plasma-on runs. However, compared with the feedstock, the residues contained less saturated, aromatic and resin fractions but more asphaltene and toluene insoluble fractions. This implies that intramolecular condensation was more significant than inter-molecular condensation, especially in the plasma-on runs. This should be attributed to the higher stabilization ability of the H-2 plasma for fragments or radicals and gas (plasma) flow by which the fragments or radicals are separated rapidly. (C) 2014 Elsevier Ltd. All rights reserved.