The main aim of this study is to investigate the effect of NO and NO2 on the combustion characteristics such as pressure development and combustion phasing in natural gas HCCI engine. A secondary aim is to demonstrate a method of obtaining a significant sensitizing effect on methane oxidation reaction from small amounts of NOx. Experiments were conducted using a rapid compression-expansion machine that was constructed from a single-cylinder diesel engine.First, the sensitizing effect of NO and NO2 on the HCCI combustion of natural gas was investigated in a case where NOx was uniformly mixed into a charge. Obtained results show that the auto-ignition timing is significantly advanced and an acute heat release is promoted by adding either NO or NO2.Second, the effect of non-uniform mixing of NOx into an air-fuel mixture was investigated according to our expectation that the existence of local spots with high NOx concentrations can assist auto-ignition more effectively than the same amount of NOx homogeneously distributed. Obtained results show that when NO and air-fuel mixture are individually inducted into a cylinder, by retarding the NO induction timing to suppress the progress of mixing, combustion phasing is advanced and higher combustion pressure is obtained with keeping the inducted NO amount constant.