Bunker fuel oil, which mainly consists of residue from crude oil refining process, is commonly used for stationary and marine diesel engines. In spite decreasing quality of bunker fuel oil during last years, higher combustion efficiency and lower emission must be achieved. Up to today, ignition quality was the only criterion for combustion characteristics of such fuels. Therefore, contrary to gas oil, formation of the fuel spray and combustion process were not examined in detail. These are clarified in this paper, based on results from engine trials and visualization of fuel spray combustion. Further, methods to improve combustion are discussed.