Particulates exhausted from diesel engine are imperfectly burned carbons of oil and are difficult to re­move. Oil is organic compounds composed of hydrogen and carbon and has relatively long chain that the numbers of carbons are from 10 to 18. When a short chain oil is used in diesel engine, the combustion efficiency could be better than that used a long chain oil, and then particulates could be reduced. In this paper, in order to increase combustion efficiency and reduce particle materials exhausted from diesel engine, an experimental works to crack the long chain oil to short chain oil by streamer plasma induced in oil is conducted. The discharge electrode geometries of a needle and a plate are tested. AC high voltage with 60 Hz of frequency is supplied for electrical discharge in oil. Fresh oil and cracked oil are analyzed with GC-MS (Gas Chromatograph-Mass). From the results, Oil cracking is observed at 14 kV, however, it can not observed below 14 kV in two reactor tested. Operating time is not significantly influence to oil cracking. With derived by the results by GC/MS, it is confirmed that a long chain oil with a heavy molecule weight was changed to a short chain oil with a light molecule weight.