Abstract: The free radicals •H,•OH,•O are important active intermediates in catenarian oxidation reaction,which favor mixing and burning of fuel vapor, consequently fuel can be saved.After diesel oil was activated by rare earth materials, the signal of hydroxylic radical became stronger.The more electrons and negative ions were produced by rare earth materials, the stronger the signal of hydroxylic radical was.The test on diesel oil that activated by rare earth materials was conducted in an engine.The results showed that diesel oil consumption rate was reduced by 0—8%,and CO,NO₂, NO exhaust emission was reduced by 6%—30%.