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A design of size-reduced 90° branch line coupler (BLC) is described in this work. A common defected ground structure (CDGS) is adopted to design the coupler. Common DGS patterns are realized on the common ground plane of double-sided microstrip lines of which ground planes are attached to each other back-to-back. The CDGS patterns play a great role in size-reduction of microwave circuits. As an example, a BLC is designed, fabricated and measured at 1GHz for the verification of the size-reduction using CDGS. The size of the fabricated coupler is less than 50% of the normal one without any critical degradation in performances. The measured S-parameters well agree with the simulation results.