Abstract: The spheroidization rate affects the properties of ductile cast iron. In this paper, ultrasonic C scanning was used to measure the velocity of ductile cast iron QT400-18 sample, and the rigidity of different parts of the sample was recorded and followed by metallographic testing for these specific positions. Afterwards, the metallographic photos were evaluated to get the spheroidization rate of graphite. The results show that the sound speed and spheroidization rate,rigidity and spheroidization rate of the casting have a good correlation. In actual production, the spheroidization rate of the product can be checked by rigidity measurement on spot, and 100% rapid tests can be carried out by ultrasonic sound velocity measurement method.