Diesel fuels having low cetane numbers may have ignition problems such as diesel knock, difficult engine starts in cold weather, and need for addition of cetane number improving agents. As organic peroxides are known to be effective cetane number improving additives, hydroperoxides produced in the autoxidation of hydrocarbons can be expected to improve cetane number. In this study, the cetane numbers of diesel fuels autoxidized in the liquid phase under various conditions have been examined. As a result, autoxidized diesel fuels have been found to have higher cetane numbers. Hydroperoxides are also found to be produced in the autoxidation of diesel fuels. It is suggested that hydroperoxides produced from diesel fuel autoxidation should have an important role in increasing its cetane number.