Abstract: Based on the self-designed plasma injector, a nonequilibrium oxygen plasma is generated by dielectric barrier discharge to study the effect of pure oxygen plasma on methane-oxygen diffusive flame. The discharge characteristics, thermal and gas dynamic effects of the plasma, together with the flame characteristic parameters, the discharge power, and the cost-benefit ratio are all experimentally analyzed by utilizing various diagnostic methods, such as schlieren imaging, thermocouple, infrared thermometer, broadband and CH* chemiluminescence imaging. Results show that the discharge current increases notably due to combustion. The current increases with the discharge voltage rises, yet it decreases with the flow rate of oxygen increases. The discharge power of oxygen plasma is higher than that of air plasma, yet the light emission intensity is obviously weaker under a certain flow rate and voltage. The heating of discharge plasma is mainly restricted within the injector. Since the maximum temperature increment led by the plasma is only 30.6 K, it is assumed that the thermal effect of oxygen plasma is too weak to influence the flame combustion. Moreover, according to the experimental results of air discharge plasma, the heat release process of the discharge reactions should mainly be determined by species, which contain oxygen. An induced jet, which has three velocity components, is generated by the discharge. The angle of the original oxygen jet is enlarged due to this induced jet, especially for a higher voltage. The relative capability of the plasma on enlarging the jet angle is stronger under a lower flow rate. The flame becomes more stable and its heat release enhances under certain conditions mainly due to the plasma gas dynamic effect, which changes the mixing between fuel and oxidizer. The lowest cost-benefit ratio of the plasma is only 2.2％ in this study, and the plasma performs better under a high flow rate or a small mixing ratio.