This study aims to investigate the combustion process and soot formation in diesel combustion with main/after injection. Heat release rates and high-speed photographs of luminous flame were analyzed changing injection interval and after-injection quantity using a constant volume vessel equipped with a multi-hole nozzle. The results show that luminous flame is still observed in the later stage of combustion for longer injection interval. Luminous flame decays earlier as the quantity of after injection decreases. With the lowest quantity of after injection, luminous flame disappears earlier than in the case of single injection.