Abstract: ASME B31G criterion is widely used assessment procedure, it simulates the profile of the corrosion defect with parabola, which leads to some conservatism. In fact the profile of the corrosion defect is very irregular. This paper uses the quartic spline function to describe and calculates the area of the corrosion defect, and uses this area to revise the B31G formula. The real examples show reasonable agreement withthe experiments.