Leg-lengthening over nail can reduce the time spent in external fixation, that would increase patient comfort and and reduce complications. The biomechanical characteristics of leg-lengthening with an external fixator over a nail are, however, not well known. The cow tibial lengthening models using a modified external fixater and an intramedullary locking nail (SIGN) in combination were tested in axial load, bending load and torsional load to evaluate the biomechanical characteristics of the modified external fixater. It was said that, the modified external fixater and SIGN nail in combination was strong enough to resist axial load, torsional load, bending load in leg-lengthening over nail.