This study examines the fundamental characteristics for a flow-induced vibration of shell-type long-span gates which have a gate front consisted of vertical and inclined weir plates. The shell-type gates possess two freedoms to vibrate in the streamwise and vertical directions due to their bending flexibility. Both streamwise and vertical vibrations couple well each other through hydrodynamical forces acting on the weir plates, thus resulting in a severe self-excited vibration. A two-dimensional model test for a shell-type long-span gate under small gate-openings was performed and a previous study reported the experimental results in detail. Here, the obtained results for the vibration frequency, the added mass of water and the dynamic press-shut gate trajectories are carefully studied. Moreover, the self-excited vibration mechanism is studied and the self-excited vibration possibility of Shell-type long-span gates under practical uses is examined. Finally a design criterion for dynamic stability is presented.