This paper discusses a method of smoke reduction for city buses with diesel engines. A mixed diesel fuel of liquified petroleum gas and the standard fuel, diesel oil, was found to reduce smokiness. Bench tests, vehicle road tests, and actual service tests were carried out. Bench test evaluations were made of smokiness, hydrocarbon, NO2, and CO emissions. Road tests evaluated exhaust smokiness, performance, fuel consumption, and running cost. Although the cost of liquified petroleum gas can be twice that of diesel oil, this increased running cost is virtually offset by the lower engine maintenance costs. The possibility of extending the use of this dual fuel to other types of bus engines is raised.