The engine performance and the exhaust gas emissions in a dual fuel compression ignition engine with natural gas as the main fuel and a small quantity of pilot injection of diesel fuel with the ultra-high injection pressure of 250 MPa as an ignition source were investigated at 0.3 MPa and 0.8 MPa IMEP. With increasing injection pressure the unburned loss decreases and the thermal efficiency improves at both IMEP conditions. At the 0.3 MPa IMEP the THC and CO emissions are significantly reduced when maintaining the equivalence ratio of natural gas with decreasing the volumetric efficiency by intake gas throttling, but the NOx emissions increase and excessive intake gas throttling results in a decrease in the indicated thermal efficiency. Under the 250 MPa pilot injection condition simultaneous reductions in the NOx, THC, and CO emissions can be established with maintaining the equivalence ratio of natural gas by intake gas throttling. Two-stage split pilot injection of diesel fuel effectively reduces the maximum rate of pressure rise and the NOx emissions without deteriorations in the thermal efficiency or the THC and CO emissions at 0.3 MPa IMEP. At 0.8 MPa IMEP, lower intake oxygen concentrations with cooled EGR and a suitable equivalence ratio of natural gas, around 0.5, with supercharging are necessary to suppress the rapid combustion as well as to establish the simultaneous reduction in the NOx, THC, and CO emissions.