Dual fuel diesel engines using compressed natural gas (CNG) are an attractive low polluting application, because natural gas is a clean low CO2 emitting fuel with superior resource availability. In dual fuel diesel engines with CNG as the main fuel the natural gas is supplied from the intake-pipe and the pre-mixture formed in the cylinder is spontaneously ignited by an injected spray of ordinary diesel fuel. Dual fuel engines of this type have the advantages that only limited engine modifications are needed and that low calorie gas fuels such as biogas can be used.To reduce NOx emissions in the dual fuel operation, the present study conducted the diesel combustion with a setup similar to that used with EGR. To dilute the intake air, the experiments used N2 or CO2 gases which are the major components of EGR. The diluent gas addition ratio was defined as the mass ratio of the supplied diluent to the intake charge which is composed of air and diluent.The results showed that the trade-off relation between the NOx and smoke emissions can be improved significantly by using N2 or CO2 gas dilution in the dual fuel diesel combustion. Compared with CO2 dilution, the dual fuel operation with N2 dilution was superior in maintaining a higher brake thermal efficiency. The major result is that compared with conventional diesel operation, the dual fuel operation at a 61% CNG supply (energy ratio) with 14.6% N2 addition (mass flow ratio) reduced the NOx and smoke (Opacity concentration) by 69% and 60% respectively, with the decreases in brake thermal efficiency less than 3%.