Acute unstable tibial shaft fractures are common in these days of high energy trauma. There are many options for this common and complicated fracture. Flexible intramedullary nailing using closed technique is prevalent in that this has much technical, functional, and physiological advantages. We reviewed 51 cases of acute tibial shaft fractures treated with flexible intramedullary nailing. There were 3 nonunions (6% ), and two refractures. Among 46 united cases, 8 cases (17 % ) showed no bridging callus formation within 20 weeks. Followings are the results of average time reaching each of radiological bony progression stage for 46 united cases. l. Average time to earliest discernible callus formation (stage I ) was 13.3 weeks (range:8-21 weeks). 2. Average time to definite and bridging callus formation (stage II ) was 18.1 weeks (range:8-32 weeks). 3. Average time to some oblitetation of fracture line (stage III) was 32.8 weeks (range:17-50 weeks). 4. Average time to complete obliteration of fracture line (stage IV ) was 14.0 months(range:12-18 months).