Interest in lignocellulosic by-products is important as they represent a cheap renewable resource with high potential for environmental friendly technologies. Regarding the applications either for bio-oil, animal feeding, paper, char or activated char production or chemicals recovering….., many treatments have been studied and optimized. These include : chemical, enzymatic, thermal, and hydrothermal treatments. Then, an important bibliography exists about the effect of the treatments on the chemical constituents of the fibers. Nevertheless, very few works relate the effect of the treatments on the mechanical properties of the fibers. Our purpose is the study of the effect of pyrolysis, alkaline and acid treatments on chemical composition, morphology and mechanical properties of the fibers. The two studied fibers, sugar cane bagasse and banana pseudo-stem fibers, present an increase of tensile strength after thermal treatment. At the contrary, the chemical attacks lead to a decrease of the strength. The preliminary results obtained for composite reinforced with the treated fibers also show best strength for composites made of pyrolyzed fibers. The mechanical results are not fully consistent with the tensile strength evolutions of fibers.