Nickel and vanadyl porphyrins were separated from atmospheric residue of Canadian oil sand bitumen by solvent extraction and column chromatography and then subjected to noncatalytic thermal process under hydrogen. The petroporphyrins before and after thermal process were characterized by UV-vis spectroscopy and positive-ion electrospray ionization FT-ICR mass spectroscopy to probe their structural transformation. Three main vanadyl porphyrins, including N4VO, N4VOS, and N4VO2 and a fraction of N5VO2 are identified in the feed fraction. With time increasing, the relative abundance of CnH2n-28N4VO (DBE = 17) increases initially and then decreases, in contrast with CnH2n-28N4VO (DBE = 18). It suggests the hydrogenation and rapid hydrogenolysis of petroporphyrins. The carbon number shifts to the lower mass range with increased process severity, indicating extensive thermal cracking reactions of petroporphyrins have occurred. N4VOS porphyrins show very similar variation of DBE and carbon number distribution as N4VO. A considerable proportion of new types of N4VO2, N4VO3 and N5VO2 are identified in the product after 30 min by accurate mass measurement and isotopic distribution. Under more severe conditions, these new species gradually diminish. It is inferred that the new species could most possibly derive from disassociation of large molecules in addition to chemical transformation. H2S and high hydrogen pressure could promote the hydrogenation of petroporphyrins. H2S can also enhance their thermal cracking reaction while high hydrogen pressure inhibits it. Nickel porphyrins present almost the same phenomena with vanadyl porphyrins, though with low content. Analysis of the petroporphyrins at the molecular level reveals their behavior and transformation during thermal process under hydrogen and could also benefit the catalysts design in HDM process.