Waste of bangkirai wood is very abundant and is not currently utilized optimally. It is known that composites using natural fibers have many advantages such as the material produced is lighter, corrosion-resistant, the availability of abundant raw materials, low production costs and natural fibers are more environmentally friendly. Natural fiber composite is a solution to the utilization of bangkirai wood waste. For optimal mechanical properties, chemical treatment of bangkirai wood fiber waste is needed. This study was conducted to determine the effect of alkalization treatment of natural fibers of bangkirai wood (Shorea Laevifolia Endert) with polyester matrix on the mechanical properties of tensile and flexural strength. The making of composites was carried out using the hand lay up method with the size of tensile test specimens that referred to ASTM D 638 and bending test specimens which referred to ASTM D 790 and from the tensile test results morphological observations were performed using Scanning Electron Microscopy (SEM).The results of this study are the optimal tensile strength values of 58.33 MPa in fiber composites with 7% alkalization treatment. As for the optimal bending strength in bending test of 65.63 MPa in fiber composites with 5% alkalization treatment.