Abstract-This paper presents a comprehensive study on the hybrid mode analysis of a periodic structure in a Suspended Microstrip and Broadside-coupled Suspended Stripline. The analysis has been use of Floquet's theorem in special harmonics to express the field equations in various sub regions of the periodic loaded suspended substrate. Their characteristic equation is derived, using the Galerkin's procedure. The unknown electric field distribution in the substrate region, corresponding to one unit cell of the periodic structure is specified in terms of suitable basis functions. The characteristic of slow wave properties, resonance behavior and the passband-stopband characteristics are at the millimeter wave frequencies as a various structural parameters are presented.