Abstract: The leakage reasons of the internally ribbed tube in a boiler water wall during operation were analyzed by methods of macroscopic examination, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis and so on. The results show that during the period of start-up, shutdown or load change of the boiler unit, due to the large temperature fluctuation, the internally ribbed tube wall on the fire-facing side would withstand a large alternating thermal stress, which resulted in transversal fatigue cracks appearing in the tube wall. At the same time, the corrosive ash containing sulfur which attached to the tube wall promoted the expansion of the cracks. The above two factors co-acted and caused the leakage of the internally ribbed tube in the water wall.