Quantitative investigation of the impedance of defected ground structure (DGS)-based microstrip lines is presented in this paper. Empirical curves and curve-fitted relations of impedances of DGS lines on RT/Duroid 5880 (εr=2.2, h=0.7874 mm) are obtained. The effect of the dumbbell slot size on impedance variations and on the operating frequency shifts are established by producing design curves that relate the DGS line and the non-DGS line characteristics and facilitate device designs. As an application, a rat-race ring coupler based on defected ground structure has been designed, realized and measured. The DGS confers size reduction due to the shift of frequency to lower values. The measurements are in a good agreement with the simulated results and emphasize the size reduction and the harmonics suppression of the DGS coupler