This study measures the effect of the inclination of the occlusal surfaces of flat teeth, on the magnitude of exerted stresses in a model of the complete upper denture (CUD), in order to estimate which inclination could be more beneficial for the stressed denture. Three groups of dentures were produced, with three different inclinations of their posterior flat teeth on the buccolingual direction: group 1 with an inward inclination of flat posterior teeth on the buccolingual direction (anti-Monson arrangement), group 2 with flat occlusion (no inclination) and group 3 with outward inclination (Monson arrangement). Using commercial edentulous molds and standardized procedures, six identical CUDs were produced for each group. Two rosette strain gauges were cemented onto the midline of each denture specimen, for measuring the stress magnitudes. The use of flat teeth with outward inclination may prolong the lifetime of the denture, by reducing the detrimental stresses (tensile principal and maximum shear stress). When flat posterior teeth with inward inclination are used, the reinforcement of CUD is necessary, to withstand the significant increase of the developed stresses (p < 0.05).