Objective To investigate the relationship between the tibia callus diameter ratio(CDR) and prognosis during tibial distraction and the occurrenceof late deformity or fracture. Methods We measured tibiallengthening callus diameter and added up the cases of angular deformity and fracture in 68 casesfrom January 1996 to December 2001, to calculated callus diameter ratios and compare the relationship between the tibia callus diameter during tibial distraction and the occurrence of late callus angular deformity or fracture. Results In 23 cases of CDRlt;80%, 13 cases had new bone fracture, 21 cases had angular deformity gt;5 degree. In 6 cases of 81%lt;CDRlt;85%, there were 4 cases of angular deformity gt;5 degree. In the other 39 cases of CDRgt;85%, there were no fracture and angular deformity. Conclusion When the CDR was gt;85%, there wereno angular deformity and fracture, but when the CDR was lt;80%, the complications of fracture and angular deformity occur. CDR is a better alarming index for preventing the complications occurring in tibial lengthening.