Fiber reinforced polymer composites are lightweight, high specific strength and stiffness, and high fatigue and corrosion resistant materials used widely as a good alternative to traditional materials in various applications. Properties of composites are strongly dependent on the interphase, a three-dimensional region of nanometer size in the vicinity of the fiber-matrix boundary that possesses properties different from those of either the fiber reinforcement or the matrix resin and governs the load transfer from matrix to fiber. There is a contradiction between achieving high interfacial shear strength and improving energy absorption by chemically modifying the interphase. The researches show that the mechanical interlocking between fiber and resin in addition to the chemical bonding in the interphase can improve the strength and energy absorption of the composite, simultaneously. The goal of this dissertation is to increase both strength and energy absorption of composite through tailoring the interphase of the glass fiber-epoxy composite. For this purpose, two ways of roughening the E-glass fiber surface have been studied in order to create the mechanical interlocking between fiber and resin. The first technique involves forming in-situ islands of texture on the glass fiber surface by using silane blends of glycidoxypropyltrimethoxy silane (GPS) and tetraethoxy silane (TEOS). The results of microdroplet test showed that the sample of G1M1 (with blend ratio of 1:1) has the highest interfacial shear strength and friction stress. Moreover, it’s specific energy absorption due to different failure modes (debonding, dynamic sliding, and quasi-static sliding) was improved 58%, 49% and 67%, respectively, compared to GPS sized fiber reinforced epoxy composite. The addition of TEOS in the silane blend significantly changes the fiber surface morphology, modifies the mode of failure and lets the fracture follow a torturous path along the interphase so that more energy is absorbed. In the second technique, the silica nanoparticles incorporated into the silane blends of GPS and MPS (3-Methacryloxypropyltrimethoxysilane). The results of microdoplet test showed that the interfacial shear strength, apparent (?app) and ultimate (?ult), of hybrid sizing (G1M1-NP sample containing of 1% nanosilica) was improved 47.5% and 42.7%, respectively, compared to mixed sizing (G1M1 sample), and 30% and 32.6%, respectively, compared to GPS sample. The hybrid sizing with the incorporation of nanoparticles, can improve the critical energy release rate (Gic), the work of adhesion (WA) and the adhesion pressure (?ult). The specific debonding energy absorption of G1M1-NP sample can be increased up to 3.4 times that of GPS and G1M1 samples. The hybrid sizing can increase the specific dynamic sliding energy absorption up to 46% and 22%, compared to GPS and G1M1 samples, respectively. Also, the specific quasi-static sliding energy absorption increased by 2 and 1.5 times than GPS and G1M1 samples, respectively. In order to validate the effect of tailored interphase structure, which is induced by creating mechanical interlocking between fiber and resin, on macroscopic composite properties, composite panels were made from different sized glass fiber and epoxy resin, and the mechanical properties of them were tested using tensile, flexural, shear, Charpy impact and punch shear tests. The results indicated that the tensile, flexural and shear strength of hybrid sizing sized glass fabric/epoxy composite panel (G1M1-NP) increase 8%, 30% and 23%, respectively, compared to the G1M1 sample. The result of impact and shear punch tests showed that the G1M1-NP composite panel exhibited improvement in the shear strength and impact energy absorption consistent with the trends in micromechanical measurements. The mechanical properties of composite reinforced with woven fabric containing of weft and warp with different sizing (G+M) also showed improvement in shear strength and impact energy absorption compared to the sample of G1M1. In general, the hybrid sizing with the incorporation of nanoparticles can greatly improve the impact resistance (i.e. energy absorption) of composites without sacrificing its structural performance (i.e. strength). The rule of mixtures, Halpin-Tsai (H-T) and Chamis equations were used to evaluate the elastic properties of textile composites. Elastic modulus values calculated according to H-T equations showed good agreement with experimental values. Both micromechanical and macromechanical tests demonstrate the significant influence of tailoring the interphase structure on improving the impact performance of the composites. Keywords: Interphase, Surface modification, Micromechanical test, Macromechanical test, Silica nanoparticles, Mechanical interlocking