Composite materials reinforced with microfibrillated cellulose (MFC) fibers were prepared and characterized in terms of their mechanical properties. The surface of the MFC fibers was treated with a silane coupling reagent having an NH 2 functional group to improve fiber–matrix adhesion. Due to the unique structure of the MFC, which consists of nano-order-scale interconnected fibrils and microfibrils with greatly expanded surface area and submicron pore size compared to conventional cellulose, it was possible to enhance the effect of silane coupling agent and thereby improve the compatibility between the fibers and matrix and also the dispersibility of fibers. The Young's modulus was significantly increased to more than 70%.