In this study, it was investigated how the addition of the boron oxide and boric acid (H3BO3) which we produce and are derivatives of the boron element, that has great importance for our country, in the polyester materials, commonly used in composite production, at different rates affected their general structure and mechanical properties. The samples were prepared using seven different compositions. These samples were formed by adding 5-10-20% boron oxide (B2O3) and 5-10-20% boric acid (H3BO3) into polyester. Then, the internal structures of these samples were examined metallographically and the differences in their structures were determined. In order to determine the strength of the samples, the tensile, compression and three-point bending tests were performed. As a result of the evaluations, it was determined that as the B2O3 amount increased in the samples, the strength value increased; as the H3BO3 increased, the resistance strength decreased.