Abstract: 3-D calibration for freehand Ultrasound (US) image is an extremely important technique to build up 3-D US image system in ultrasound Non-destructive Testing (NDT). Calibration is a procedure to calculate the spatial transformation matrix, spatial relationship between the US image plane and the tracker attached to the US probe. In this research, a different cross-string phantom and the corresponding algorithm are investigated. The strings and crosses out of the scanning plane in the phantom accelerate interactive operation speed, guiding the operator to find the scanning plane quickly. Furthermore the ten crosses in the scanning plane provide the coordinates and spatial vectors for the calibration algorithm, thus the different calibration algorithm based on the least-squares fitting method of the homologous points matching can be realized. The results show that the US probe positioning time is no more than 5 sec. The precision and the accuracy results show that the algorithm calculates more accurate calibration matrix than that is obtained through the other published methods in the same operating time. The results solve the obligatory pre-procedure for computerize 3-D freehand ultrasonic in NDT.