In this study, improving the interfacial adhesion of epoxy / polyethylene fiber (UHMWPE) composites by grafting glycidyl methacrylate (GMA) on the surface of the fiber has been investigated. For grafting process, design of experiments was used and two factors of monomer concentration and reaction time were selected as the process variables. After grafting process, the results of grafting percentage has been analyzed. Then, the ATR-IR test has been done to approve the presence of GMA groups on polyethylene chains. After that, changes in interfacial adhesion was measured by pull-out test to determine the effect caused by surface modification of fibers. Also, to evaluate the effect of surface modification process on mechanical properties of composites, samples of unidirectional UHMWPE fiber/epoxy composites were prepared and tested by tensile and bending methods. Then, tensile fracture surfaces of samples were studied by scanning electron microscopy. The results showed that the grafting percent increases by increasing monomer concentration and reaction time. However, the effect of monomer concentration on grafting percentage is clearly greater than the reaction time. In addition, a quantitative relation between grafting percentage and two factors of monomer concentration and reaction time were obtained using the software. In pull-out test, Interfacial Shear Strength (IFSS) of epoxy / treated fibers was increased. The IFSS of epoxy and fibers grafted with 11%, 25% and 40% GMA, is respectively 126%, 195% and 220% of untreated fibers. The results of mechanical tests showed that in samples made with 11% grafted fibers, tensile strength and work up to break were increased 10% and 36% compared to untreated fibers, but in samples with 25% and 40% grafted fibers, tensile strength and work up to break decreased. In the bending test, flexural strength and flexural modulus of samples made from 11% and 25% grafted fibers didn't change considerably compared to the samples made from untreated fibers, but in samples made from 40% grafted fibers, reduction in fiber strength and flexural modulus is observed. Meanwhile, all macroscopic and microscopic fracture surfaces of tensile and bending test samples show that fracture behavior of composites goes to brittle fracture by increasing the grafting percent. Keywords: UHMWPE fiber, epoxy matrix, surface treatment, GMA, pull-out test, mechanical properties