Abstract: The fracture failure analysis was carried out on the two lengths of fractured and cracked welded 16Mn steel penstock used for hydroelectric power plant with reference to the chemical composition and physical property of the standard steel plate of GB1591-88 for low alloy structural steel;the technique of thermal simulation for welding was adopted to evaluate the notch sensitivity,impact toughness and thermal strain embrittlement tendency of the heat affected zone; through the welding crack test of inclined Y groove,physical inspection of mechanical property of welded joint,etc,the steel plate quality and welding process were reviewed as well.