Resonator structures have been proposed and modified as a method of reducing the group velocity of light for optical buffering in telecommunication field. This makes the characterization and optimization of the method become important to apply. In this paper, the characterization of vertically coupler resonator is modified by investigating its geometrical parameters. The modified models with conical rod portion as the slow light structure is described where the group velocity parameters are varied. The results found that the separation gap between fibers, lg and the rod radius, a are significant in reducing the group velocity of light through such a system in the case of lossless resonators.