Abstract: In a power station, leakage failure occurred to a heat transfer tube of the nuclear island cooling system frequently. The leaked heat transfer tube was made up of red copper, and the leak points lay on the welding seam between the elbow tube and straight tube. The leakage reasons were studied by chemical composition analysis, metallographic examination, electron microscope and energy spectrum analysis, X-ray diffraction analysis and so on. The results show that:the basic metal of the copper tube didn't suffer damage; the leakage path developed alone the welding seam, and the width of the leakage path in the inner tube was wider than that in the outer tube; the failure was originated from the defect of the welding seam in inner tube; the excess of phosphorous resulted in the defect in the welding seam, and plenty of bubbles existed in the welding seam, which were the main reasons of the leakage.