Abstract: A cut off valve cracked during the hydraulic test, resulting in leakage. The causes of the valve cracking were analyzed by means of macroscopic observation, metallographic examination, hardness test and fracture analysis. The results show that the material of the flange and valve body of the cut off valve were gray cast iron, which was not consistent with the surface identification of the valve. The graphite in the gray cast iron was C-shaped, and the graphite length was long, which resulted in the poor mechanical properties of the material. At the same time, the matrix structure of the cut off valve was ferrite, and only a small amount of pearlite was distributed along the edge of the strip graphite, which resulted in the toughness degradation of the material. The crack initiated and expanded at the stress concentration of the valve body and flange connection, and finally formed penetrating crack under the action of large stress, resulting in leakage.