Two dimensional solid phononic crystals such as plates, shells can be widely applied to the control of vibration and noise as a kind of structural materials. It is very significant to study the band gaps in two dimensional phononic crystals to apply them in the vibration and noise control for structures and as a filter for acoustical signal. The multiple-scattering theory is integrated with wave equations to study the elastic wave band gaps in two-dimensional phononic crystals. The carbon/proxy system is used as numerical example. The results show that a full band gap is presented when carbon cylinder with high propagation velocity is arranged periodically in proxy with low propagation velocity.