An effective highly dispersive medium is proposed by utilizing strong dispersion of localized defect mode in a microstrip photonic crystal. Linewidth narrowing in a composite microstrip resonator using this photonic-crystal-based effective medium is investigated and its dependence on the structure parameters is presented. Both numerical simulations and experimental results demonstrate that compared to the traditional resonator, the Q-factor increases much more quickly and the peak transmission decreases more slowly at the same time for the composite resonator.