This paper systemically evaluates the different reinforcement roles of carbon nanotubes in those nanocomposites with different matrix stiffness while the curing process is controlled. Both mechanical test and microscope observation indicated that such reinforcement would gradually reduce while increasing the stiffness of matrix. However in the soft and ductile composites, carbon nanotubes show a significant reinforcement without fracture strain decreasing. The interface interaction is poor between carbon nanotubes and matrix in the stiff composite, and therefore, they have little contribution to the mechanical properties of composite. This research may help to propose a further positive solution for designing and fabricating carbon nanotube–epoxy nanocomposites.