The discharge of methane/air mixture gas had been investigated by using argon as carrier gas under atmospheric pressure for the first time. The characteristics of the discharge and the emission spectra of the plasma were gained to reveal the properties of ignition and combustion of the mixture gas assisted by plasma under lean-burn condition. Based on the parallel-plate electrode radio frequency (13.56 MHz) dielectric barrier discharge plasma generator, the discharge behavior of the gas mixture, with volume fraction of 90% argon and 10% air, was studied by experiments under atmospheric pressure. A stable and uniform discharge was achieved when methane had been added into the mixture gas at stoichiometric ratio (Φ＝1), while the content of argon maintained at 90%. At last, six discharge experiments were made at different level of the equivalent ratio (0.4 to 1.9). To identify the radicals generated during the process, the data of the emission spectrum were recorded by the spectrometer under different discharge conditions. By using a program compiled by the authors for the nitrogen second positive band system simulation, comparison between the experimental and simulated spectra of band (0-2) 380.4 nm, (1-3) 375.4 nm was used to determine the rotational temperature of nitrogen molecules. The rotational temperature is considered as the temperature of discharge gas. Research results show that the discharging gas temperature would reach 1 150 K in the case of 90%argon/10%air; the temperature would rise to 1 390 K when the mixture was added stoichiometric air and methane; in the case of different equivalence ratios, the plasma gas temperature would go up at the range of 70～240 K, against the mixture of 90%Argon/10%air; The existence of some active radicals, such as CH/H/OH/CH2O, and the raise of temperature indicated that plasma chemical reaction occurred and the release of chemical heat.