A new type of dual fuel supply system has been developed. This system is able to economically convert conventional diesel engines into dual-fuel engines like LPG/Diesel engines and CNG/Diesel engines, which are capable of either using single diesel fuel or using dual-fuel including both diesel and CNG fuel or both diesel and LPG fuel.These diesel-LPG engines have been applied to the diesel buses in the public transportation of Guangzhou city, one of the biggest cities in China, owning to their low soot emissions, excellent operating performances and extremely low cost as well.Compared with the diesel baseline engine, it was found that there were a significant reduction in soot emission and an improvement of the fuel consumption with the diesel-LPG engine. Also the strategy on LPG content is discussed in order to meet the demands for soot emission, fuel economy, transient performance and output power at the same time.The power output from the diesel-LPG engine has not been reduced but slightly improved, though the volumetric efficiency is lower on the dual-fuel mode than on the diesel mode. It was found that there was hardly any effect of diesel-LPG fuel the power output of the engine when LPG content was as high as 40 percent approximately.The experimental results of exhaust pollutants including NOx, CO and HC emissions are presented in the paper. In general, the NOx emission was slightly reduced when diesel-LPG fuel was used in comparison with net diesel fuel. Hydrocarbon emission, however, was quite high on the diesel-LPG mode, especially at partial loads with higher LPG content. The carbon monoxide (CO) emission was also increased at partial loads with higher LPG content, even though the CO emissions from diesel-LPG engine was not increased at full load.