Abstract: The seamless steel tube for high pressure boiler bursted while the pressure test was performed. The bursting reason was analyzed by means of macroscopic and microscopic analysis, mechanical properties test, chemical compositions analysis and metallographic analysis etc. The results show that there was an internal surface folding (ISF) defect along the longitudinal direction of the steel tube at the bursting position. The ISF defect resulted in the reduction of effective loaded thickness of the steel tube and the stress concentration, which was the immediate cause for the seamless steel tube busting when pressure test.