The aim of this study was to evaluate the effect of implant diameter and taper on the stress distribution pattern in a full arch screw retained cantilevered fixed mandibular restoration. Materials and methods: A simulated mandible is prepared to receive four interforaminal dental implants of diameters 3.2, 3.7, 4.7 and 6mm diameter separately. Every diameter exists in two different tapers (2&5) together with the cylindrical form to give rise to twelve models. The implants received full arch screw retained prosthesis. The material properties for compact bone, spongy bone, titanium alloy, and acrylic were identified. The nature of contact between the implant and the surrounding bone was identified as slip contact to simulate the immediate loading condition. Two loads were applied namely vertical load, and oblique load on the right premolars and first molar with magnitude 200N. The model was restrained at its inferior border to avoid total body displacement of the mandible. The models were meshed with a fine solid mesh and the analysis conducted with the iteration method. Results were collected and tabulated. Results The presence of cantilevers resulted in accentuation of bone stresses especially, at the bone distal to the terminal implant. Increasing implant diameter decreased stresses transmitted to the bone and to the decrease in implant micro motion which leads to better initial stability. Increasing implant taper led to increase the transmitted stresses to the bone and cylindrical implants showed the least stresses values followed by 2degrees then the 5 degrees taper .and to the increase in implant micro motion . Conclusion: increasing implant diameter and decreasing implant taper lead to better stress distribution in immediate loading condition and led to decrease in implant displacement with subsequent improvement in initial stability.