Common rail injection system is a flexible equipment to control injection timing, pressure and multi-injection. Many studies and applications have reported the advantages of using common rail system to meet the strict emission regulation and to improve engine performance for diesel engine by this flexibility. The main objective of this study is to investigate the effect of after-fuel-injection strategies on engine performance and emissions. The study was carried out on a single cylinder optical direct injection diesel engine equipped with a high pressure common rail fuel injection system. The after-injection was found to be very effective in completing the oxidation process and reducing the particulate emissions even when small fuel quantity was injected.