A photonic crystal (PC) is an artificial medium with a periodic structure stacked by alternating two or more different materials with distinct refractive indices. It is known that there exist some photonic band gaps (PBGs) in a PC. Wide PBGs are usually needed in photonic applications. In this thesis, we investigate the band gap extension for a one-dimensional ternary PC, in which each period is made of a metal layer sandwiched by two dielectric layers. Two topics will be studied. First, it can be seen that the addition of metal layer will strongly enhance the PBG compared to that without metal layer, i.e., the dielectric-dielectric photonic crystal (DDPC). Next, we investigate the effective plasma frequency for such a ternary MDPC. We find that the effective plasma frequency increases with the increase in the thickness of the metal layer. All the above analyses are made based on the Abeles theory which is an elegant method in dealing with the multilayer system.