The effect of fuel injection timing relative to ignition timing on natural gas direct-injection combustion was studied by using a rapid compression machine (RCM). The ignition timing was fixed at 80 ms after the compression start. When the injection timing was relatively early (injection start at 60 ms), the heat release pattern showed a slower burn in the initial stage and a faster burn in the late stage, which is similar to that of flame propagation of a premixed gas. In contrast to this, when the injection timing was relatively late (injection start at 75 ms), the heat release rate showed a faster burn in the initial stage and a slower burn in the late stage, which is similar to that of diesel combustion. The shortest duration was realized at the injection end timing of 80 ms (the same timing as the ignition timing) over a wide range of equivalence ratio. The degree of charge stratification and the intensity of turbulence generated by the fuel jet are considered to cause this behavior. Early injection leads to longer duration of the initial combustion, whereas late injection leads to a longer duration of the late combustion. Early injection showed relatively lower CO concentration in the combustion products while late injection gave relatively lower NOx. It was suggested that early injection leads to combustion with weaker stratification, and late injection leads to combustion with stronger stratification. Combustion efficiency was kept at a high value over a wide range of equivalence ratio.