Natural gas is one of the most promising alternative fuels for automotive vehicles. However, composition of natural gas varies between the originating fields and may be further modified due to processing and additional mixing. These variations are known to affect engine performance and emissions through changes in fuel metering and combustion characteristics.In the present study, it was examined the effect of gas composition on vehicle performance such as fuel economy, driveability and exhaust emissions. Analysis was made of using 3 types of NGVs which were manufactured by auto maker and 6 different fuels which were selected in consideration of the variation in fuel composition on the worldwide market. These results may be utilized to develop natural gas engine in auto maker and/or to establish the fuel standard in the refuelling station.