This study investigated the plasma reforming process for diesel focusing on the relative ratio of oxygen to fuel. Excess O 2 in the partial oxidation process is known to increase the combustion portion, resulting in a decreased yield of H 2 and CO. However, in this parametric investigation, there was no apparent decrease in the H 2 and CO selectivity. Adding O 2 did not increase the portion of combustion in the overall reaction. Rather, an excess O 2 supply from partial oxidation stoichiometry resulted in an increase in CO 2 selectivity without a reduction in CO selectivity. Heavy hydrocarbon species were identified as a source of CO 2 in excess O 2 conditions due to preferential oxidative cracking. The additional oxidation of C 1 –C 4 species by excess O 2 provided a minor contribution to CO 2 . Excess O 2 affects soot generation characteristics by suppressing the agglomeration of soot particles, resulting in smaller particle generation. However, the oxidation of soot particles does not provide a major contribution to increasing the CO 2 selectivity. The results show that in a real reforming process, controlling the O 2 supply does not have a strong effect on the process selectivity of hydrogen.