In this paper, we developed a microscopic approach to understand rubber reinforcement using coarse-grained molecular dynamics simulations. We investigated static uniaxial tensile behavior of filled and unfilled rubber models, and found two reinforcement mechanisms. One of these is the same as the mechanism predicted by Guth, which depends only on the volume content of fillers. We have confirmed this mechanism at small strain region. The other is caused by filler-filler network created by the advantage of chemical bond at large strain. In this region, some polymers linked fillers and were stretched, that is, these polymers generated tension. Additionally, we investigated the effect of filler distribution on rubber reinforcement.