Abstract: It was found that cracking occurred on the inner wall of the socket welding branch outlet in the evacuation pipe and the location near the socket of the header pipe of reheat steam pipe in a power plant boiler during overhaul after service for over 100,000 h. The causes of cracking were analyzed by macro- and micro-morphology observation, chemical composition analysis, mechanical property test, microstructure observation, and micro-component analysis. The results show that the structure design of the evacuation pipe of the reheat steam pipe was unreasonable, leading to the accumulation of a large amount of condensate in the horizontal pipe section in front of the first valve of the evacuation pipe back end. Thermal fatigue cracks initiated at the inner wall of the socket welding branch outlet in evacuation pipe, inner wall at down stream part of the header pipe close to the socket and inner wall of the branch outlet of reheat steam pipe under the combined action of countercurrent condensate and high temperature steam. Under the combined effect of steam pressure, cyclic thermal stress and corrosive carbonate substances, thermal fatigue cracks expanded, eventually resulting in the cracking of the pipe.