In this paper, an approach is taken to reduce the size and suppress harmonics of rat race coupler with the help of Circular defected ground structure. The reduction in size is possible due to the lowering of resonant frequency which in turn is a result of slow wave effect. The proposed structure is additionally able to make the response as a wide band. The two parameters of CDGS are modeled rigorously to obtain the achieved response. First, the conventional rat race coupler is studied and later the effect of CDGS on the same structure is analyzed. © 2016 IEEE.