This paper is concerned with the emulsification of residual fuel to improve the combustion of marine diesel engines.The experiment was conducted to investigate the effective water particles size and the stability of emulsified fuel, and the effects of water particles size and water content ratio which were dispersed in fuel on engine performances and burning characteristics were investigated by means of a marine diesel engine.It has been found that the minuteness and homogeneity of water particles dispersed in fuel are the most important factors for applying emulsified fuel to marine diessel engines.