In this research, the effect of multi-walled carbon nanotubes functionalized with carboxylic acids having different diameters and lengths on the mechanical properties of E-glass/epoxy laminated composites was investigated. Multi-walled carbon nanotubes at 0.1, 0.3 and 0.5 wt% ratios were added to the epoxy. The tensile strength, the modulus of elasticity, Poisson's ratio in the fiber direction and the tensile strength, the modulus of elasticity in the transverse direction of composites were determined. In addition, the shear strength, the flexural strength and the interlaminar shear strength were investigated. According to the obtained results, the shear strength and tensile strength in both directions of the composites decreased. The modulus of elasticity in the fiber direction of the composites increased 16.01% at the ratio of 0.3 wt% of the long-thin nanotubes. The Poisson's ratio and the modulus of elasticity in the transverse direction of the composites enhanced 16.67% and 33.25% at the ratio of 0.5 wt% of the long-thin nanotubes, respectively. The flexural strength and the interlaminar shear strength of the composites improved 34.75% and 19.72% at the ratio of 0.1 wt% of the short-thin nanotubes, respectively.