To reveal the mechanism of plasma (assisted the ignition process of methane/air further, schematic of dielectric barrier discharge plasma system with atmospheric air was designed and set up, the emission spectrum of dielectric barrier discharge plasma with atmospheric air was measured, and the active particles produced by the interaction of dielectric barrier discharge plasma with atmospheric air were analyzed with the spectrum technology, the ignition model and calculation methods of sensitivity analysis and reaction path analysis were given, effects of NO and O3 on the ignition delay time were simulated, and the chemical kinetics mechanism of NO andO3 assisted ignition was revealed via sensitivity analysis and reaction path analysis. The results show that main excited particles of N2 and O3 are generated via effect of plasma on the atmospheric air, which are converted into active particles of NOx and O3 in the end, the life of which are longer than any other active particles, effects of plasma on the ignition is simplified as effects of NOx and O3 on the ignition; NO and O3 could reduce the ignition delay time significantly, but the amplitude decrease with increase of the initial temperature, this is because the rate of ignition is decided by the oxidation rate of CH3, the oxidized pathway of CH3 is R155 and R156 for auto-ignition and their rates are slower when temperature is low, so the ignition delay time of methane/air is longer; NO could reduce the ignition delay time significantly because of the oxidized pathway of CH3 is changed to R327 CH3O2+NO=CH3O+NO2, R328 CH3+NO2=CH3O+NO for NOx(assisted ignition process from R155 and R156 for auto-ignition; and the chemical kinetic effect is the dominating factor of O3 on the ignition and which change the reaction path.