Abstract: For the thickness measurement of high pressure and high temperature tube, a waveguide bar with the same material was suggested to be vertically welded to the pipe thickness monitoring position with full penetration, in which the waveguide might be cooled by air or water to the temperature making thickness monitoring possible for pipes with conventional ultrasonic sensor and coupling medium. Taking stainless steel as example, waveguides were welded to circular flat plate, straight pipe and elbow for thickness measuring. The experiment results were in agreement with the specimen thicknesses. The relationship between the sound velocity and temperature was worked out for the thickness measuring system and was used for correction of ultrasonic thickness gauge readings, which verified the feasibility of the method.