Abstract: The high strength bolts used for tower fastening in a wind power plant, appeared multiple fracture phenomena at the thread position after a month's use. By means of macro examination, chemical composition analysis, mechanical property test, metallographic examination, fracture analysis and so on, the fracture reasons of the bolts were analyzed. The results show that the surface total decarburization layer depth of the failure high strength bolts at the thread position exceeded the standard requirement, and at the same time the decarburization layer depth was very uneven. Stress concentration easily formed at the deepest total decarburization position, and the decarburization also caused the decrease of strength and fatigue property of the bolts at local positions and made the bolts not bear the design load. So stress cracks appeared here and resulted in the final fatigue fracture failure of the high strength bolts.