PURPOSE: The purpose of this study is for the rigid fixation of the pelvic ring by quantifying and comparing the extraction strength of cancellous screws in the sacral ala and body. MATERIALS AND METHODS: Six cadaveric human pelvis were obtained for test of the extraction strengths of three groups of 7.0mm cannulated cancellous screws: shortthreaded in the sacral ala, short-threaded in the sacral body, long-threaded in the sacral body. The extraction strengths of these groups were compared with each other. RESULTS: The mean extraction strengths of short-threaded screws in the sacral ala, short-threaded screws in the sacral body and long-threaded screws in the sacral body were 10.26N, 25.85N and 48.37N respectively. The mean extraction strength of the long-threaded screws in the body was significantly greater than that of the shortthreaded screws in the ala and body. The mean extraction strength of the short-threaded screws in the body was greater than that of the short-threaded screws in the ala, but insignificant statistically. CONCLUSION: In choosing iliosacral lag screws to stabilize the posterior pelvic ring disruption, superior fixation is achieved by inserting the long-threaded screw in the sacral body.