For the final goal of developing the natural gas fueled spark ignition engine with high thermal efficiency and low pollutant emission, the effects of the fluid flow inside a combustion chamber on the combustion process of a homogeneous lean mixture of natural gas and air were examined using a rapid compression combustor. The rapid compression combustor was designed to simulate the combustion process in a spark ignition engine involving the rapid compression of a mixture and the heal release during flame propagation. The main advantage of using this combustor is that experiments can be made under the idealized and well-controlled conditions. The time history of pressure in the combustion cylinder was measured with a pressure transducer.The fluid flow in the combustion cylinder was varied using two kinds of experimental technique. First, compression ratio, piston speed and the configuration of the piston head were changed. These changes attain different flow fields formed by piston motion. The results showed that the configuration of the piston head affected the combustion duration and that the other factors had weak effects on it. Secondly, the mixture of natural gas and air was injected into the combustion cylinder before the compression stroke, and the direction of the injection and the time interval between the end of mixture charge and the start of compression stroke were changed. In this test, the location of spark ignition was also changed and the flame propagation was photographically observed. Moreover, the two-dimensional velocity distribution of the swirl flow was measured with particle image velocimetry. As expected, the combustion duration decreased with increases in the mean velocity and the turbulence intensity of the fluid flow. If the intense swirl flow existed in the combustion chamber, the combustion duration was minimized with spark ignition at the eccentric location.