An experimental study was made to investigate the effect of combustion chamber configuration, top clearance, nominal swirl ratio, and spark plug position on in-cylinder combustion in a spark-ignited natural gas engine, which is converted from a direct injection diesel engine. Flame propagation in a single-cylinder visualization engine was measured from the cylinder axis direction by the high speed schlieren method, over the wide range of combustion chamber configuration, top clearance, nominal swirl ratio, and spark plug position. The results showed that flame does not propagate concentrically to the spark plug, but is shifted by swirl, which is the main flow in this engine. Smaller piston cavity diameter led to more rapid flame propagation, resulting in larger heat release rate and larger cylinder pressure. Piston cavity diameter does not affect the initial combustion until TDC. Top clearance had little effect on flame propagation, resulting in little difference in flame propagation speed, heat release rate and cylinder pressure. Larger nominal swirl ratio led to more rapid flame propagation, resulting in larger heat release rate and larger cylinder pressure. Nominal swirl ratio affects flame propagation speed more than it affects the field speed, resulting in smaller shift of flame propagation by swirl as nominal swirl ratio increases. Farther spark plug position to cylinder center led to more rapid flame propagation, resulting in larger heat release rate and larger cylinder pressure. At the farthest spark plug position, however, flame propagation speed, heat release rate, and cylinder pressure decreases.