Abstract: The emission behavior of Ar plasma generated by atmospheric pressure coaxial microwave plasma torch was studied using optical emission spectroscopy. The variations of specific emission intensity and intensity ratio with microwave power and gas flow rate were analyzed to investigate the relationship between emission behavior and plasma properties. It was found that the dependence of emission behavior on input microwave power is mainly determined by the changes in electron density and electron temperature in the plasma. On the other hand, the variation of atomic number density of the source neutral gas with different gas flow rates also has a significant impact on the emission. When analyzing the variation of emission intensity with plasma axial length at the optimum operating state, it was found that the emission behavior of Ar plasma exhibits a single peak while N₂ plasma shows double peaks. The physical principles behind these behaviors are discussed in this study. The results can be used for characterizing atmospheric pressure microwave plasma and improving aerosol analysis systems.