Objective: This study aimed to investigate biomechanically the effects of onlay fibula grafts on Vancouver Type B1 Periprosthetic Femoral Fractures (PPFs). Methods: Vancouver Type B1 PFF models were created in 25 fourth-generation synthetic femurs and fixed with locking plates using bicortical, unicortical screws, and cables. While no graft was used in group 1, onlay fibula grafts were placed anteriorly in group 2 and medially in group 3. In group 4, the cortical strut allograft was placed on the medial femoral cortex, and a locking compression plate (LCP) was applied to the lateral femoral cortex. In group 5, the strut allograft was placed over the anterior cortex of the femur and fixed with the same technique as in group 4. All models were then subjected to rotational and axial cyclical stiffness tests and load to failure to measure and compare the mechanical strengths of the constructs. Results: The mean stiffness values of group 4 with medial allograft, before and after cyclical loading, were higher than all other groups, under both rotational and axial forces. The mean stiffness values of fibula autografts (groups 2 and 3) were similar to that of anterior allografts (group 5) in each test except that the mean initial axial stiffness of group 5 was higher than group 2. Failure loads were also not different between the groups. Conclusion: Although the rigidity of Vancouver type B1 periprosthetic femur fractures is highest if allografts are placed medially, fibula autografts can also provide similar fixation strengths to allografts if locking plates with unicortical and bicortical screws and cables are used.