The electron temperature of the Ar/CH₄ plasma was diagnosed under the pressure of 40~80 Pa and the microwave power of 400~800 W by Optical Emission Spectrometry （OES）. The experimental results show that the electron temperature obtained by the OES is between 0.75 eV and 4 eV under the above experimental conditions. Through measurement of Ar or CH₄ plasma， it is feasible to use the OES method in microwave coaxial plasma with carbon-containing gas. This research can further expand the application of the OES method in the PECVD field.