PURPOSE: To investigate the proper insertion site for a tibial intramedullary nail in the Korean. MATERIALS AND METHODS: Forty volunteers without trauma below knee joint were studied to determine relationship between central axis of medullary canal and lateral tibial spine, patellar tendon and the proper insertion site, and to evaluation changes of proper insertion site during rotation of knee and effective diameter. RESULTS: The proper insertion site located average 4.3+/-0.9 mm medial to the lateral tibial spine. The proper insertion site was in the medial 1/3 of patellar tendon in 6 knees, middle 1/3 of patellar tendon in 20 knees, and lateral 1/3 of patellar tendon in 14 knees. The proper insertion site might be changed two times in internal rotation more than in external rotation. The effective diameter was more narrow in medial slope than in lateral slope at proximal tibia. CONCLUSION: The study indicates the ideal insertion site of tibial nail is the medial aspect of the lateral tibial spine in Korean. To reduce the mal-reduction from a improper insertion site, lateral insertion site might be safer than medial one. However, individual variations in the relationship between the patellar tendon and tibial medullary canal should be considered.