Planar couplers have been commonly used in microwave systems for measuring transmitted and reflected power with accuracy. They have several advantages, such as easy fabrication, repeat ability, and low cost. Extensive research has been conducted on the design of planar couplers due to their widespread application. In this thesis, investigations have been carried out to study the effect of DGS in the design of planar coupler. The applications of DGS such as size reduction, harmonic reduction, coupling enhancement, etc. are investigated in this thesis. The phenomenon of size reduction due to slow wave effect and suppression of unwanted Spurious responses due to band stop characteristics have been explained. Application of DGS to enhance the coupling and directivity are also examined.