The Diesel Dual Fuel (DDF) vehicle is one of the technologies to convert diesel vehicles for natural gas usage. The purpose of this research was to study the possibility of a DDF vehicle to meet emission standards for diesel vehicles. This research was done for small passenger vehicles and commercial vehicles. The exhaust emissions compliance of such vehicles in a New European Driving Cycle (NEDC) mode which was composed of Urban Driving Cycles (UDC) and an Extra Urban Driving Cycle (EUDC) was evaluated. (see APPENDIXFigure A1) In this study, the passenger vehicle engine, compliant with the EURO4 standard, was converted to a DDF engine.Engine bench tests under steady state conditions showed similar result to previous papers. Total hydrocarbon (HC) emission was extremely high, compared to diesel engine. The NEDC mode emissions of the DDF vehicle were estimated based on these engine bench test results. The results indicate that the HC + nitrogen oxides (NOx) emission of the DDF is approximately six times higher than the EURO4 regulation and it doesn't meet the EURO4 standard due to the high HC emission.Then, combustion was improved to reduce HC emission. By increasing the air-fuel mixture temperature and intake airflow, HC emission was improved by 45% However, HC emission must be reduced by 83% or more in order to meet the EURO4 HC+NOx emission standard, so the reduction by combustion improvement is still insufficient.It was concluded that further significant reduction through combustion improvements would be difficult, and other methods for meeting the EURO4 regulations were studied. Narrowing DDF area, operating with diesel fuel only in low load condition where large amount of HC emission emitted, was evaluated although it reduces Natural Gas (NG) usage. As a result, the NG fuel usage rate in NEDC mode was approximately 20%, and alternating diesel fuel to NG fuel was limited. However, limiting the DDF operation area may be effective for suburban driving that often uses the high-load condition and in long-distance transport trucks.Although a less thermal efficiency was expected, a stoichiometric combustion with a three-way catalyst to reduce HC emission in the DDF engine was investigated. The simulated mode emissions showed potential to meet the EURO4 standards with this method. Although the diesel-fuel-equivalent fuel consumption (FC) of DDF worsened as expected and increased by approximately 13% over the fuel consumption during diesel fuel use, an HC emission reduction and an NG fuel usage rate of at least 50% were achieved. Therefore, DDF engine stoichiometric combustion with a three-way catalyst seems to be an effective technology in countries where the price of NG fuel is inexpensive.