Objective To explore the application value of self-made tibial mechanical axis locator in tibial extra-articular deformity in total knee arthroplasty (TKA) for improving the lower extremity force line. Methods Between January and August 2012, 13 cases (21 knees) of osteoarthritis with tibial extra-articular deformity were treated, including 5 males (8 knees) and 8 females (13 knees) with an average age of 66.5 years (range, 58-78 years). The disease duration was 2-5 years (mean, 3.5 years). The knee society score (KSS) was 45.5 ± 15.5. Extra-articular deformities included 1 case of knee valgus (2 knees) and 12 cases of knee varus (19 knees). Preoperative full-length X-ray films of lower extremities showed 10-21° valgus or varus deformity of tibial extra joint. Self-made tibial mechanical axis locator was used to determine and mark coronal tibial mechanical axis under X-ray before TKA, and then osteotomy was performed with extramedullary positioning device according to the mechanical axis marker. Results All incisions healed by first intention, without related complications of infection and joint instability. All patients were followed up 5-12 months (mean, 8.3 months). The X-ray examination showed lt; 2° knee deviation angle in the others except 1 case of 2.9° knee deviation angle at 3 days after operation, and the accurate rate was 95.2%. No loosening or instability of prosthesis occurred during follow-up. KSS score was 85.5 ± 15.0 at last follow-up, showing significant difference when compared with preoperative score (t=12.82, P=0.00). Conclusion The seft-made tibial mechanical axis locator can improve the accurate rate of the lower extremity force line in TKA for tibia extra-articular deformity.