The objective of this study is to reveal the hydroelastic behavior of the cross deck structure of a high speed catamaran subjected to slamming load. An analytical method using flat plate approximation is proposed to solve the slamming problem of the two dimensional structures where the hydroelastic effects as well as the pile up water on the free surface are accounted for. Dynamic responses of simply supported seven-span panels penetrating the wave crest are analyzed. It is found that the maximum deflection and bending moment at the location of initial hit tend to be proportional to the impact velocity as far as the wave profile is relatively flat and the impact velocity is high enough. The results are similar to those obtained by Faltinsen et al. A simple equation to estimate the maximum deflection of the panel is obtained by considering the transformation of fluid kinetic energy into the strain energy.