One semi EV vulcanization system was applied for observing the effects of nano-sized montmorillonite on vulcanization and reinforcement properties of natural rubber (NR) composites. The montmorillonite as a nano-sized reinforcing filler was added into NR at varied concentrations i.e 2.0, 4.0, 6.0, 8.0 and 10.0 parts per hundred rubber (phr). It was found that the montmorillonite functioned as a co-curing material and reinforcing filler. It decreased the optimum cure times of the NR composites. The higher the montmorillonite concentration, the shorter the optimum cure times were. The montmorillonite also increased the torque difference, tensile moduli, reinforcement index and tensile strength. The tensile strength was improved up to an 8.0 phr. of montmorillonite concentration.