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 2 GHz, the size of the proposed coupler is 2.3 cm2.7 cm, 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.