Abstract: This paper presents a new method of diagnosing arc plasma by infra-red photography microcompurer image processing.Experiments have shown that this method is correct,reliable,sinple,fast,visual,and helpful to real-time measurement and control.The high frequency pulsed TIG welding arc was first diagnosed by this method.Certain new information about the high frequency pulsed arc was obtained.A new approach to the study of the high frequency arc and the mechanism of "hig hfrequency effect" was revealed.