The oxidation mechanism of diesel particulate matter (PM, carbon is the main component) in plasma discharges is investigated by using X-ray photoelectron spectroscopy (XPS) and thermogravimetry. XPS analysis results showed that the ratio of sp(2) carbon decreased and the ratio of sp(3) carbon increased after plasma discharge; while the ratio of sp(2) carbon increased and the ratio of sp(3) carbon decreased after reacting in O-3 atmosphere without plasma discharge.