Flexible intramedullary nails are commonly used to treat femoral fractures in children. This study evaluated the biomechanical differences between stainless steel and titanium nails when securing transverse and comminuted fractures in a synthetic femur model. Retrograde flexible stainless steel and titanium nails placed in a divergent "C" pattern were mechanically tested, and axial rotation and compression stiffness were analyzed with a two-way ANOVA. Rotational stability was significantly greater for titanium nails than stainless steel nails for both fracture patterns. Axial compression stiffness was significantly greater for titanium nails than stainless steel nails for both fracture patterns. There was no statistical difference between materials for axial "failure" load that produced 5 mm of shortening. Titanium intramedullary nails were more stable than stainless steel nails in torsion and axial compression. Both materials stabilized simulated fractures at levels beyond physiologic non-weight-bearing loads without permanent deformation.