In this study, the effect of epoxy matrix modification with different CNFs loadings consisting of 0.5, 1.0 and 1.5 wt. % on the mechanical properties and interfacial shear strength (IFSS) of epoxy/ultra-high molecular weight polyethylene (UHMWPE) fiber composites was investigated. In this study, UHMWPE fiber surface was treated with GMA as much as 5%. Then, the success of the grafting was proved with attenuated total reflectance-infra red (ATR-IR). As the fiber was treated, the effect of surface treatment on morphology, roughness and mechanical properties of UHMWPE fiber was evaluated by scanning electron microscopy, atomic force microscopy and tensile tests, respectively. Along with modifying fibers, epoxy resin was modified with carbon nanofibers and dispersion of carbon nanofibers in the resin was investigated by optical microscopy. Afterwards, in order to investigate changes in surface wetting and adhesion between the fiber and matrix, micro bond (Pull-out) test and contact angle measurement of the different samples were accomplished. Tensile and Flexural test on manufactured two-phase UHMWPE fiber/epoxy composites and three-phase UHMWPE fiber/epoxy/ carbon nanofibers nanocomposites were also carried out in order to investigate the effect of fiber and matrix modification on their mechanical properties. At last, the tensile fracture surfaces of composites and nanocomposites were studied by scanning electron microscopy. The results showed that the chemical process of grafting glycidyl methacrylate on UHMWPE fiber surface increased the fiber surface roughness and decrease in filament strength was observed. Microbond test results revealed that the simultaneous modification of fiber surface with 5 wt. % of GMA monomer and matrix with 0.5 wt. % of carbon nanofibers was considered as the maximum amount of improvement as much as 319 percent. By comparing microbond and contact angle results, synergistic effect of fiber treatment and matrix modification on improving interfacial adhesion was proved and the highest IFSS and lowest contact angle were observed as the fiber and matrix were treated at the same time. Mechanical test results showed that the maximum improvement of mechanical properties was detected for the samples in which the fiber and matrix were modified concurrently. By comparing test results of mechanical properties, it was predicted that the fiber treatment had a good control on tensile properties and the matrix modification had a great influence on the flexural properties. According to the mechanical test results, the maximum improvement in tensile and flexural modulus for nanocomposites containing 1.5 wt. % carbon nanofibers were 27.9 and 40.6 %, respectively. On the other hand, the maximum improvements of the order 52.3 and 69% for tensile and flexural strength of samples containing 1 wt. % carbon nanofibers were observed. Furthermore, microscopic examination of tensile fracture surfaces revealed that fracture mechanism in unmodified composites turned to a brittle fracture state in three-phase by applying synchronized modifications on fiber and epoxy resin. Key words: UHMWPE fiber, epoxy resin, glycidyl methacrylate, carbon nanofibers, interfacial shear strength, mechanical properties