Injection-molded short-fiber-reinforced thermoplastics are widely used in today’s industry. Nevertheless, their mechanical behavior is difficult to model, especially because of strong anisotropy induced by complex fiber distributions of orientation. Moreover, the intrinsic variability of plant fibers’ properties leads to an even more complex behavior than with mineral fibers and therefore increase the uncertainty for behavior prediction of these materials. The aim of this review is to provide basic and more specific knowledge about dealing with the uncertainty related to injection molded short-plant-fiber-reinforced thermoplastics behavior, focusing on variability induced by both injection-molded process and natural variability of plant fibers properties. To achieve this goal, it is important to understand the behavior of SFRT before considering the uncertainty induced by the use of natural fibers. Thus, in the first place, the authors have chosen to limit the sources of uncertainty related to fibers by studying the case of a short-glass-fibers-reinforced thermoplastic. Then, after discussing the sources of uncertainty related to the use of natural fibers, the methods for the quantification, the propagation and the management of uncertainties are analyzed.