Thermal plasmas are expected to be utilized for a number of industrial applications, such as nanomaterial synthesis and waste treatment. The thermal plasma generation and its characteristics remain to be explored for more attractive processing. In particular, the electrode dynamics are one of the most considerable issues for the practical use of industrial applications, because it determines the electrode lifetime and the performance of materials in thermal plasmas. High-speed imaging provides a powerful tool to reveal the plasma dynamics. High-speed video camera with bandpass filter system was used to measure the electrode temperature as well as the arc fluctuation. Electrode temperature was evaluated from the radiation intensities at 785 nm and 880 nm from the electrode thermal emission. This system can be also applied to observe the dynamic behavior of vapor generation from the electrode on the millisecond time scale. Emissions from tungsten vapor were observed at the wavelength of 393 nm. The tungsten electrode started to evaporate just after the peak top of the arc current in the anodic period. The tungsten vapor became the main species in the anodic arc when the electrode started to evaporate. In contrast, a small amount of tungsten evaporation was observed during the cathodic period. These experimental studies enable us to understand the electrode erosion mechanism of the multiphase AC arc. Fundamental researches for electrode phenomena determining thermal plasma characteristics lead to the development of innovative industrial applications of material processing and waste treatment.