Purpose: The purpose of this study is to evaluate the effect of dynamization which was done for the treatment of delayed union or persisting fracture gap after interlocking intramedullary nailing, by radiographic and physical examination. Materials and Methods: We analyzed 22 cases these were treated with dynamization from 247 cases of tibia shaft fracture treated initially with static interlocking intramedullary nailing from February l990 to May 2000 and were followed up more than lyear. The result of dynamization was classified as 3 groups and we divided each results 3 categories that is effective result, uncertain result and ineffective result. Group A is cases that achived ultimate union with shortening of fracture gap and was 10 cases. Group B is that achieved ultimate union but fracture gap did not reduced and was 10 cases. Group C is that showed ultimate non-union and was 2 cases. Results: The result is that dynamization was effective only 4 cases of group A. 6 cases of group A and 4 cases of group B showed instability(radiolucent halo around nail, hypertrophied callus and leg pain), and 6 cases of group B showed no shortening of fracture gap, thus these were classified as uncertain result. Group C(2 cases) showed non-union, classified as ineffective. Conclusion: Dynamization caused instability in most tibia fractures except simple mid-shaft fracture. So in the comminuted fractures and distal or proximal l/3 oblique fractures, other procedures such as bone graft, refixation should be considered rather than dynamization.