Abstract: Servo valve bolt of electrohydraulic oil of high regulating valve of small steam turbine in supercritical unit in a power plant fractured after working for 3.5 a. The fracture cause was analyzed by methods such as chemical composition analysis, hardness testing, metallographic analysis and fracture analysis. The results show that the fracture property of the bolt was fatigue fracture, and the fracture presented multi-source characteristics. There were many serious manufacturing machining defects in the thread of the bolt. Fatigue cracks initiated in these defects and propagated under complex fatigue load. The cracks became unstable when propagated to critical size, leading to the instantaneous fracture. The static load on the other bolts increased signi ficantly after the fatigue-fracture of one bolt, leading to the instant fracture under overload of the other bolts.