Abstract: The bursting reasons of boiler water wall tubes during operation were analyzed by means of chemical compositions analysis, metallographic examination, hardness testing and scanning electronic microscope analysis. The results show that the analyzed two bursted tubes were caused by short term over-heating and long term over-temperature separately. Because the high temperature acting time and degree of short term over-heating and long term over-temperature were different, the expanding deformation, bursting fracture and microstructure of the two bursted tubes were different.