Purpose: This study investigated the effect of implant length, diameter, and surface contact on the stresses developed in a fibular free flap. Materials and Methods: Finite element models for dental implants placed in a patient-specific fibula were created using a patient-specific fibula computed tomography scan and geometry files of commercially available dental implants. The finite element models involved nine dental implants of different lengths and diameters: 3.5, 4.3, and 5.0 mm in diameter and 10.0, 11.5, and 13.0 mm in length. Three contact conditions between the implant and the fibular flap were investigated: complete fusion, friction, and smooth contact, representing complete osseointegration, a rough implant-bone interface, and no osseointegration, respectively. Finite element analysis was performed to examine the average von Mises stresses around the local implant-fibula interface within the fibula under a load of 500 N along the long axis of the implant and posterior-anterior and medial-lateral directions. Results: Both the level of osseointegration and implant size had noticeable effects on the mechanical stress inside the fibula. The stress introduced to the fibula gradually decreased as the implant osseointegrated into the bone. An optimal implant size existed where the internal stresses were minimized; this trend was seen when investigating both the implant diameter and length. In this study, an implant with a diameter of 4.3 mm and length of 10 mm produced the lowest mechanical stresses overall. Conclusion: Both implant length and diameter were influential; stresses were seen to decrease to a minimum then subsequently increase as either dimension increased. Additionally, stresses in bone introduced by an implant decreased as the degree of interaction between the implant and fibula increased. Complete fusion between the implant and bone yielded the lowest stresses.