This study conducted several tensile tests to determine the effect of 20-30 nm silicon dioxide nanopowder on the mechanical properties of composite material polyester/carbon fiber. Samples were prepared at weight fractions of carbon fibers (i.e. 25, 40, and 55%), with different weights of silica nanoparticles (i.e. 0.16, 0.2, and 0.24%). The experimental results showed that the mechanical properties improved at various ratios as a result of increasing the weight fraction of the carbon fibers and the ratio of the silicon dioxide nanopowder in the composition of the composite samples. The maximum increase by 33.49% resulting from increasing the weight fraction from 25% to 40% at 0.16% silicon dioxide nanopowder. The maximum effect of increasing the weight of the silicon dioxide nanopowder from 0.2 to 0.24 resulted from increasing the stress by 33.53% at weight fraction of 25%. The SEM images of the structure showed the distribution of nanoparticles and crack growth in the region neighboring the fracture after the tensile test at different weight fractions of carbon fibers and nano-silica particles. The improvement in the mechanical properties of this low-cost composite material when using nanomaterials has potential for use in multiple applications, including boat hulls.