PURPOSE: The purpose was to evaluate fragment reduction feasilibty when applying extension block Kirschner-wire technique for bony mallet finger. METHODS: We treated 48 displaced mallet finger fractures by a two extension block Kirschner-wire technique. Among these operation group, we found dorsal rotation of fragment in 18 cases, making it difficult to get anatomical reduction. The patients were divided into two groups. One group of 30 patients did not show dorsal rotation of fragment and anatomical reduction was achieved easily. Another group of 18 patients showed dorsal rotation of fragment and additional methods was applied to achieve anatomical reduction. RESULTS: Joint surface involvement was significant greater in groups showing dorsal rotation of fragment than group which did not show (57.1% and 49.7%, respectively) (p=0.01). The groups whose joint surface involvement more than 50% had higher risk of dorsal rotation of fragment than the group less than 50%, with the odds ratio of 6.11. CONCLUSION: We could encounter the cases which showed dorsal rotation of the fracture fragment when treating the bony mallet finger with extension block K-wire technique especially the joint surface involvement was more than 50%. So if we can evaluate the extents of joint surface involvement and prepare additional method preoperatively when dorsal rotation of fragment is expected, it is possible to get more favorable results.