Carbon nanotubes have shown the superior mechanical, electrical and thermal properties. These outstanding properties as well as a high aspect ratio and low density make carbon nanotubes an ideal reinforcement to develop superior nanocomposites. Mechanical tests indicated that the reinforcement role of carbon nanotubes is affected by the stiffness matrix. It is well known that the degree of cure of the epoxy has great influence on their mechanical properties. In this investigation, the matrix stiffness is controlled by changing the mixture ratio between the epoxy and hardener. Two different contents (1% wt. and 2% wt.) of the multi-wall carbon nanotubes in the epoxy are proposed in this work. Tensile tests are conducted to determine the mechanical properties of the nanocomposites, including the Young’s modulus, yield stress, tensile strength and fracture strain. Experimental results show that the mechanical properties are increasing with the increase of the addition of multi-wall carbon nanotubes. The reinforcement role of the multi-wall carbon nanotubes is decreasing while increasing the stiffness matrix.