The effect of the composition of propane (C₃H₈) and butane (C₄H₁₀) in liquefied petroleum gas (LPG) was investigated in a dual-fuel HCCI engine fueled with di-methyl ether (DME) and LPG. The composition of LPG affects DME-LPG dual fuel HCCI combustion due to the difference in the physical properties of propane that and butane such as octane number, auto-ignition temperature and heat of vaporization.DME was injected directly into the cylinder at various injection timing from 160 to 350 crank angle degrees (CAD). LPG was injected at the intake port with a fixed injection timing at 20 CAD.It was found that power output was increased with propane ratio. This gain in power output resulted from increased expansion work due to the better anti-knock properties of propane. However, higher propane ratio made combustion efficiency decrease because of the suppression in low temperature reaction of DME which determines heat release amount of high temperature reaction.Carbon monoxide (CO) emission was reduced and nitrogen oxide (NOx) emission increased as the propane ratio increased. This was because increased in-cylinder pressure and temperature, caused by the prolonged ignition delay, induced oxidation of CO and formation of NOx. However, due to the chain termination steps that occurred with the addition of LPG, hydrocarbon (HC) emission was increased as the propane ratio increased even though the in-cylinder temperature was increased as well.Given these findings pure propane may be the best fuel composition for a DME-LPG HCCI engine when considering engine performance. However, it should be noted that the use of pure propane is accompanied by slight increases in HC and NOx emissions.