With increasing restrictions on tailpipe emissions from vehicles powered by internal combustion engines and growing concern over the use of liquid hydrocarbon fuels, alternative fuels have gained popularity. Natural gas (NG) is a promising alternative fuel to diesel for road transport vehicles. However, as the chemical composition of NG varies widely with geographical location, weather and other factors, such differences in the fuel properties affect combustion characteristics. The effects of NG compositions on dual-fuel (DF) combustion under.. constant-volume conditions were studied numerically using a three-dimensional model. Parameters such as combustion pressure, temperature, fuel species and flame propagation with respect to time were analysed. The numerical model gives combustion pressure traces, which compare well with the limited experimental results from the constant-volume combustion bomb (CVCB) specifically developed for this application. The predicted results show that an increase of ethane in the NG leads to an increase in the burning rate. Moreover, the higher the amount of methane, the lower the burning rate of the NG-air mixture.