Abstract Purpose: This study aims to evaluate the effect of lengthening speed on the quality of callus and complications during distraction osteogenesis and describe an optimal lengthening speed in patients with congenital pseudarthrosis of tibia (CPT). Methods: Twenty-seven patients with CPT with a minimum follow-up of 36 months who underwent limb lengthening surgery between 1997 and 2016 with external fixator only were included in this study. All patients underwent lengthening procedures after achieving complete bone union in this study. Regenerate quality is evaluated according to the Li classification on the X-ray taken one month after the end of the distraction period. Complications were noted in post-operative follow-up period. Receiver operator characteristics (ROC) curve analysis was performed to obtain optimal lengthening speed for these patients. Result: The mean age at the time of surgery was 5.74 years. The mean lengthening speed was 0.596 mm/day. Follow-up period of 136.14 months with a mean lengthening period of 92.4 days. Mean amount of lengthening was 5.44 cm for patients with CPT. Total rate of callus with good morphological quality was calculated as 66%. According to ROC analysis, optimal cut-off values of lengthening speed for the obtaining good morphological quality callus was 0.564 mm/day for tibial lengthening in CPT. There was a significantly positive correlation between complication rate and lengthening speed for each group. Conclusion: We recommend a mean lengthening rate of 0.56mm/day for the lengthening procedures with external fixator in patients with CPT who had complete bone union at the area of pseudarthrosis. Keywords: Congenital short extremity; Congenital tibia pseudarthrosis; Lengthening rhythm; Quality of callus.