An experimental study of diesel exhaust cleaning by means of the plasma chemical pretreatment of fuel is described. Some portion of the fuel was activated in an arc discharge and turned into hydrogen-rich synthesis gas. Plasma chemical reformation of fuel was carried out by using a DC arc plasmatron that was fabricated to increase the ability of gas activation. The yield of diesel fuel reformation reached about 80-100% when small quantities of fuel (flow rate up to ~6 ml/min) were reformed. The synthesis gas, containing H2+CO, was supplied into the engine together with the rest of the fuel-air mixture, and the NOx content in its emissions reduced up to 23%.