Abstract: The high-strength bolts, which were applied to the 3 MW Wind turbine blades in a wind power plant, were fractured frequently after running for 1.5 years. The reasons of the fracture were analyzed by means of chemical composition analysis, mechanical properties testing, metallographic analysis, fracture surface analysis, and so on. The results show that the main reasons of the fracture of the bolts were lots of silicon oxide inclusions in the material, bulky grains caused by improper heat treatment, and so on. In addition, the pretightening force was relatively dispersive in the installation and maintenance process, which led to fatigue fracture failure of the bolts under the complicated stress.