Dual fuel combustion with premixed natural gas as the main fuel and diesel fuel as the ignition source was investigated in a 0.83 L, single cylinder, DI diesel engine. At low loads, increasing the equivalence ratio of natural gas to around 0.5 with intake throttling makes it possible to reduce the THC and CO emissions as well as to improve the thermal efficiency. At high loads, increasing the boost pressure moderates the combustion, but increases the THC and CO emissions, resulting in deterioration of the thermal efficiency. The EGR is essential to suppress the rapid combustion. As misfiring occurs with a compression ratio of 14.5 and there is excessively rapid combustion with 18.5 compression ratio, 16.5 is a suitable compression ratio. The combustion process can be divided into three stages: the first rapid combustion of the diesel fuel and the entrainment of natural gas mixture in the diesel fuel spray; the second mild combustion with flame propagation of the natural gas mixture; and the third a rapid combustion stage termed PREMIER (premixed mixture ignition in the end-gas region) combustion; generally PREMIER combustion occurs occasionally at several operating conditions. With PREMIER combustion, reductions in the unburned emissions as well as improvements in the thermal efficiency are feasible. The unburned gas temperatures at the onset of PREMIER combustion calculated with the two-zone model decrease with increases in the in-cylinder pressures and the equivalence ratios of natural gas, showing that the PREMIER combustion is caused by auto-ignition of the unburned natural gas mixture.