Natural gas is a promising alternative fuel that can be used to satisfy the strict engine emission regulations in many countries. To develop natural-gas engines, low emission, efficient fuel consumption, and increased power ratings have to be realized. In this study, a cylindrical constant-volume combustion chamber (CVCC) was used to investigate natural-gas combustion characteristics for different prechamber design parameters and equivalence ratios. In particular, the maximum combustion pressure and mass fraction of the burned gas were evaluated by considering orifice diameter, volume ratio of prechamber and equivalence ratio. Using this result and by analyzing the changes in combustion characteristics with variations in design parameters, the optimum prechamber parameters were determined.