Purpose: This in vitro study determined the stress distribution around an implant placed in a posterior edentulous maxillary model with simulated sinus grafts that had different degrees of stiffness. Materials and Methods: The composite photoelastic model with a standard threaded implant consisted of simulated crestal cortical, cancellous, sinus cortical, and grafted bone. The graft maturation process and inherent graft quality were represented in the model by varying the stiffness of the graft. Prior to placement of the simulated graft, axial and inclined loads were applied to the implant. The stresses that developed in the supporting structures were analyzed photoelastically. The graft was then placed and the testing procedure was repeated over 4 consecutive days, during which time the simulated graft stiffened. Results: The stress analysis indicated that before placement of the simulated graft, loading on the implant transferred the highest stresses to cortical bone. The presence of the simulated graft transferred stress from the native bone simulants to the simulated grafted bone. Discussion: As the stiffness of the graft increased, a more equitable stress distribution was observed in the multilayer bone surrounding the implant. Conclusion: Loading of an implant in a less stiff grafted sinus could lead to overloading of the native bone as well as the maturing grafted bone.