The paper is focused on the stress field acting on a continuously variable transmission (CVT) mechanism used on the high displacement scooters produced by Piaggio & C. S.p.A. The most important results of the analysis have been extrapolated with the aim of providing the designers with some guidelines useful to reduce the design error occurrence. In detail, in the paper is described the behavior of the belt and of the driven pulley, that are the critical parts of the assembly. The analysis has been conducted with a full MultiBody model of the mechanism combined with the Finite Element analysis of both the belt and the pulley. The output data so obtained have been used in a fatigue analysis in order to define the reliability of these parts. This paper could serve as a base to define a new proportioning method, that should be based on the whole stress history of each part of the assembly, computed with the aid of some numerical tools.