The estimation of emission factors (EFs) is the basis of accurate emission inventory. However, the EFs of air pollutants for motor vehicles vary under different operating conditions, which will cause uncertainty in developing emission inventory. Natural gas (NG), considered as a “cleaner” fuel than gasoline, is increasingly being used to reduce combustion emissions, but the information is scarce about how much emission reduction can be achieved for motor vehicles burning NG (NGVs) under real road driving conditions, which is necessary for evaluating the environmental benefits for NGVs. In this thesis, on-board measurements of the emissions from nine bi-fuel vehicles were conducted under different operating conditions on the real road. A comparative study was performed for the EFs of carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) for each operating condition when the vehicles using gasoline or compressed NG (CNG) as fuel.