Attempts have been made to improve exhaust gas characteristics of the direct injection diesel engine under dual-fuel operation with gas oil and LPG. In dual-fuel operation with LPG as a sub-fuel, two kinds of LPG mixing methods can be employed. One is LPG mixing in gaseous state with the intake air, which is so-called "fumigation". The other is LPG mixing in liquid state with gas oil prior to injection. The experimental results show that the mixing method in liquid state is better than the fumigation method in terms of NO and THC emission and specific energy consumption. In the mixing method in liquid state, NO is reduced in comparison with the base engine without the increase in THC, smoke and specific energy consumption if a proper squish intensity is employed.