Abstract: An ultrasonic TOFD(time of flight diffraction) B-scan image was processed in order to accurately locate crack tip in heavy aluminum butt weld. SAFT(synthetic aperture focusing technique) was introduced for improving lateral resolution of the image. Accord-ing to the geometric relation between the probes and crack tip, an al-gorithmic model for SAFT processing was founded and SAFT recon-struted image was obtained. Linearization was proposed in order to enhance time resolution of the image before SAFT processing, and a novel technique named as L-SAFT(linearization-SAFT) was devel-oped for ultrasonic TOFD B-scan image reconstruction. The results show that the technique can enhance resolution of the image effectively. Both lateral and vertical location of the crack tip in the speci-men can be measured rapidly and accurately with this techinque, which contribute to precise locating and sizing of defect.