The use of natural fibers in composite materials of plant origin involves environmental, social and economic factors that must be met. Moreover, there is a shortage of progressive energy and mineral resources in general. Leading the search for renewable raw materials and low cost, which has been the factor of significant importance in directing research involving the use of natural fibers in composites. The paper presents polyester matrix composites reinforced by short sisal fibers, randomly arranged in the mold without using technological stages of production. The resin used was polyester terephthalic pre-accelerated in relation curing agent/resin of 0,33% by volume. The sisal fibers were acquired in the local market of Belem of Para. The fibers were manually cut in lengths of 5, 10 and 15 mm, untreated and at room temperature. The fibers were characterized mechanical, physically and microstructurally. The composites were made by molding by hand without pressure and room temperature by varying the length of the fibers and different proportions between the constituents in the relationship fiber length/mass fraction (5 mm/5,41%), (10 mm/4,87%) and (15 mm/3,90%). The composites were evaluated in tensile test and evaluation of the fractured surface by scanning electron microscopy (SEM) to determine the dominant failure mechanisms for each series of composites produced. The sisal fibers showed strength (453,62 MPa) and density (1,42 g/cm₃). For composite sisal 5 mm tensile strength was 16,98 MPa, for the composite sisal 10 mm tensile strength was 22,78 MPa and the composites sisal 15 mm tensile strength was 27,05 MPa. For composite sisal greater resistance to mechanical failure mechanisms dominating was the breaking of the fibers, sisal the composites of lower resistance to mechanical failure mechanisms were predominant the pull out (tearing of the fibers) and the detachment of the fiber matrix.