Abstract: Abstract: A non-thermal plasma (NTP) technique has currently been investigated intensively as a promising technology for a diesel engine after treatment to abate PM, HC and NOx. However, there is little research about NTP technique treating the aldehyde and ketone emissions of a diesel engine. In this study, Direct Non-thermal Plasma (DNTP) technique is employed to treat the single cylinder diesel exhaust. The variation of the specific aldehyde and ketone emissions of a diesel engine at four loads before and after the treatment of DNTP is studied with 2, 4-dinitrophenylhydrazine (DNPH) derivatization method and a high performance liquid chromatography (HPLC) analysis technique. The results show that 13 kinds of aldehyde and ketone emissions could be detected under this experimental condition. Before the treatment of DNTP, with the growth of the diesel load, the specific aldehyde and ketone emissions decrease first and then increase. The specific emissions of formaldehyde are 83.22, 15.26, 25.06, 27.58 mg/(kW·h) and at each load are the largest. The specific emissions of acetaldehyde are 12.03, 3.47, 8.92, 19.97 mg/(kW·h). The minimum specific emissions of acrolein and acetone, propionaldehyde, crotonaldehyde, butyraldehyde and butanone, methacrolein, benzaldehyde, valeraldehyde are 0.43, 0.78, 0.96, 0, 0.25, 0.28, 0.23 mg/(kW·h) respectively at 50% load. The minimum specific emissions of p-tolualdehyde and hexaldehyde are 0.08, 0.04 mg/(kW·h) respectively at 75% load. After the treatment of DNTP, the specific aldehyde and ketone emissions decreases first and then increases with the growth of the diesel load, but the trend becomes mild and the specific aldehyde and ketone emissions reduce remarkably. Total removal efficiency of aldehyde and ketone emissions could reach 93.8% at 25% load, 50% load, and 75% load. Removal efficiency of acrolein and acetone, butyraldehyde and 2-butanone could reach almost 100%. At the same time, total ozone formation potentials decrease dramatically. These results provide a reference for reducing the diesel engine exhaust emissions with a DNTP technique.