Abstract: The paper presents the concept of on-line emulsification system on diesel engine. The key part (float-type-chambered hydro-diesel proportioner) CES was designed for the system. Test showed that diesel engine's existing fuel oil supply system (mainly the high pressure section) has a good emulsification effect. An on-line system extensively exploiting diesel engine's inherent emulsification function dose not need specific power-input to on-line emulsification unit any more. Such a situation obviously favors the spread of oil-emulsification technology in practical use. Test also showed that emulsifier can help maintain the stability of hydro-diesel mixture. And on-line emulsification system can not function properly without emulsifier's presence. But emulsifier's dosage is much lower than expected.