Abstract: Abstract: Biodiesel is a clean-burning alternative fuel produced from renewable resources, such as vegetable oils or animal fats. Therefore, biodiesels are becoming increasingly important due to their ease in adaptation. Biodiesel also contributes to slowing up the depletion of fossil resources and reducing the emissions from the transport sector, both environmental bene?ts and prospect in energy security. Biodiesels with higher unsaturated esters content with a higher tendency to lose stability through degradation are susceptible to oxidative degradation due to autoxidation in the presence of oxygen. Antioxidant addition is a prospective solution to the problem of improving the oxidation stability of biodiesel. Biodiesel contains a lot of unsaturated fats monoester, which will generate short chain alkenes, alkynes under high temperature cracking reaction. The concentration of H has a great influence on the decomposition path of unsaturated fat monoester and the breakdown products in the process of reaction. The mechanism of adding antioxidant includes that H is supplied by the phenol hydroxyl (OH) end the transmission of free radical chain reaction. Also the phenol hydroxyl-supplied H promotes the reduction reaction and inhibits oxidation reaction. As a result, addition of antioxidants to biodiesel could promote the reductive reactions and inhibit the oxidative reactions, which could increase the emission of soot, HC and CO but decrease NOX emission.The aim of the present study is to investigate engine performance and the exhaust emissions of a diesel engine using oxidative stabilized biodiesel. The biodiesel fuel used in the study was prepared from waste edible oils. It's an experimental investigation of the effect of antioxidant addition to biodiesel on emission characteristics. In this experiment, mark rosemary and tea polyphenol as K1 and K2. Add 8‰ of K1 and K2 into the diesel blend (B20) respectively, and mark the 2 kinds of blending fuel as K1B20 and K2B20. Test the emission characteristics of 4 kinds of fuel on 186F diesel engines, and each diesel engine was used to carry out tests under conditions of constant load and varying speed. At the rated speed of 3000r/min, on 10%, 25%, 50%, 75%, and 100% load conditions, compare the HC, CO, NOX, and soot emissions of the 4 kinds of fuel. Results show that, B20 reduced CO, HC, and soot emissions, and raised NOX emissions compared with diesel. K1B20 raised CO emission, HC emission, and smoke value respectively by 1.2%, 10.2%, and 8.5% compared with B20. K2B20 raised CO emission, HC emission, and smoke value respectively by 6.4%, 3.8%, and 4.3% compared with B20. K1B20 and K2B20 reduced NOX respectively by 9.5% and 4.2% compared with B20. The emission levels of the three pollutants were lower than those of diesel. The power and brake-speci?c fuel consumption of biodiesel fuel with antioxidant have no obvious difference from the biodiesel without added antioxidant. Thus, B20 blends with added antioxidant can be used in diesel engines without any modi?cations. Adding antioxidants can effectively reduce the NOx emissions of biodiesel and have little influence on CO, HC, and soot emissions.