In this paper, the elaboration process of a new composite material based on polyester resin reinforced with long Alfa fibres is described. The used Alfa fibres have been obtained using the chemical method of extraction based on alkali treatment with different percentage of sodium hydroxide (NaOH). The obtained average diameters of fibres treated with 9%, 10%, 11% and 14% NaOH concentrations are about 145 ± 35 µm, 90 ± 15 µm, 83 ± 15 µm and 75 ± 15 µm, respectively. The composite was elaborated with impregnation of the fabric Alfa fibres in polyester resin. Besides, an experimental characterization using tensile tests has been conducted to determine the mechanical properties of the fibres obtained with the different NaOH concentrations. The results show that the composite made of polyester resin reinforced with fibres treated with 9% concentration of NaOH presents the greatest tensile strength.