Based on provided convenient design equations for slow wave transmission lines and metamaterial line, a very compact rat-race hybrid coupler is proposed using three slow wave lines and one metamaterial line. At the design frequency of 2GHz, the size of the proposed coupler is 2.3cm × 2.7cm, which is a 74% reduction compared with the conventional one. Despite the considerable size reduction, the theoretical bandwidths based on |S₁₁|, |S₃₁|, ∠S₂₁-∠S₃₁, and ∠S₂₄-∠S₃₄, have been improved by 9%, 7%, 31%, and 59%, respectively. The measured performances are in reasonable agreement with the theoretical ones.