In-situ quantitative measurements are performed in a 300 kW DC magnetically-rotating linear plasma torch to investigate thermal characteristics of arc plasma and electrode erosion by utilizing optical emission spectroscopy. Time-resolved gas temperatures and mole fraction of copper concentrations are measured to evaluate the state of arc plasma and cathode erosion at different operation currents and magnetic fields. The erosion mass rates of the cathode by the OES method are in good accordance with the measurements by traditional weighing method over the operation range, demonstrating validity and accuracy for the developed OES method. The erosion rates of cathode for CO2 plasma are lower than that for air plasma due to carbon element covering in the inner surface of the electrode by the SEM images. Detailed discussions of sensitivity and uncertainty by the OES method are given for air plasma and for CO2 plasma.