PURPOSE: To evaluate the usefulness of early posterior internal fixation(I/F) by cannulated cancellous screw(CCS) in unstable pelvic bone fractures involving the sacrum and sacroiliac joint. MATERIALS AND METHODS: Sixteen cases were evaluated retrospectively. Classification according to the injury mechanism, using Young's criteria, was as follows: 7 anteroposterior compression, 4 lateral compression, 4 vertical shear and 1 combined mechanism. With respect to treatment, 9 cases were treated by combined anterior external fixation(E/F) and posterior I/F with CCS(7 cases) and transiliac rod(2 cases). Five cases were treated by anterior E/F only, and 2 cases were treated without surgery. We analyzed clinical results using Matta's criteria, and radiologic abnormalities. RESULTS: The management by posterior I/F with CCS and transiliac rod including anterior E/F has shown superior clinical results(7 satisfactory, 2 unsatisfactory) over the management by only anterior E/F(1 satisfactory, 4 unsatisfactory) or conservative methods(2 unsatisfactory). The rate of malunion and nonunion was also low in the former method in the radiographic analysis. CONCLUSION: Posterior I/F using CCS is an excellent surgical procedure in the treatment of unstable pelvic bone fracture due to its lessened invasiveness, early applicability, simple surgical technique and its direct effects on the prevention of several complications.