In this paper, the mechanical properties of the carbon nanotube (CNT) or alumina (Al 2O 3) nanoparticles (NPs)-modified epoxy composites were investigated experimentally. Composite dog-bone tensile specimens were prepared and tested according to ASTM D638-14 standards with CNT or Al 2O 3 nanoparticle contents of neat, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75 and 2.0% of the total resin weight. In order to compare the effect of nanoparticle type and content on the mechanical properties, tensile stress–strain curves were drawn for each parameter. Also, the ultimate tensile strength (UTS), Young’s modulus and toughness values were obtained and compared. Considerable improvements of the mechanical properties were observed by adding NPs. The results indicated that the UTS, Young’s modulus, and toughness values reached a maximum with an increment of 27.6, 18.7 and 187.9%, respectively, at Al 2O 3 nanoparticle content of 1.0 wt%, according to neat epoxy. Due to lower cost and good mechanical properties of the Al 2O 3 nanoparticle-modified composites, the Al 2O 3 nanoparticle can be used as a reinforced particle for epoxy composites. On the other hand, it was observed that the CNTs were not as effective as Al 2O 3 nanoparticles.