The removal of diesel exhausts by non-thermal plasma technology has been investigated focused on the simulation gas experiment at atmospheric pressure and room temperature for many years. However, the research for real diesel engine is need to be further studied. In the experiments, a non-thermal plasma (NTP) reactor was designed based upon dielectric barrier discharge. NO/NO2 conversion was studied as a function of the specific input energy (SIE) by varying the frequency and voltage applied on the NTP reactor through bench test. Results showed that NTP SIE was increased with the voltage at each frequency. The conversion of NO to NO2 was increased at higher NTP SIE. However, NO will further be converted to other active species but not only NO2.