Abstarct: Abstract In this research, the effect of aluminum oxide nanoparticles on the tensile and bending behavior of composites with phenolic matrix, reinforced with glass fibers, which were exposed to water, has been investigated experimentally. To make glass/phenolic composites, phenolic resin IL-800, hardener H002, bidirectional glass fibers 400 grams and aluminum oxide nanoparticles produced by US Nano Company in the amount of %0/25 by weight were used. The composite plates were made by manual porcelain laxyer method and baked under the pressure of the composite mold plates in the oven, and then beam-shaped samples were cut from it with the help of laser cutting. Composite beams have a rectangular cross-section and are subjected to bending force, tensile force and moisture absorption. The results show an increase in bending strength and tensile strength and preventing the destructive effect of moisture absorption on glass/phenolic composites by adding %0/25 by weight of aluminum oxide nanoparticles to the baxse material. baxsed on the results of the experimental tests, moisture absorption has a less destructive effect on the tensile strength and bending strength of glass/phenolic composite samples containing %0/25 by weight of aluminum oxide nanoparticles. Also, in order to investigate the distribution of aluminum oxide nanoparticles in the background and its effect on the strength and adhesion between the background material and the fibers in the glass/phenolic composites, the cross-sectional surface of the samples was imaged by a scanning electron microscope.