The ultrasonic time of flight diffraction (TOFD) is a new non destructive testing method. It measures and determines size of the defect based on the signal diffracted from tips of the defects. Because the transmission time of the diffraction signal change as the probe position changes, the endpoint of dot defect and line defect, in the TOFD D scan and B scan, will form an arc shape which is peculiar in TOFD. Because its shape is similar to hyperbola, it is also called hyperbolic fitting pointer. Based on the mathematical model, which is put forward according to the characteristics of hyperbolic fitting pointer, an algorithm of the hyperbolic fitting pointer and realization with MFC is designed and implemented.Hyperbolic fitting pointer can position and quantitatively calculate the weld defects.