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A novel miniaturized forward-wave directional coupler with periodical mushroom-shaped ground plane is proposed. The coupler can be designed with identical characteristic impedances for even and odd mode and enhanced the difference of propagation constants between the even and odd mode. These distinct propagation characteristics can be predicted by the equivalent-circuit model of a unit-cell using the Bloch-Floquet theorem. A tested sample is designed and fabricated on the FR4 substrate. We designed a 0-dB coupler with the length about 1.28 λg, and 1.0-dB coupling is measured at 2.9 GHz due to the loss. Compared with previously studies, the proposed coupler can be implemented to attain the highest coupling level with a smaller size.