Recently certain resonant phenomena in an array of cylinders in waves have been reported. At some specific frequencies, wave surface elevations between adjacent cylinders or pressures acting on each cylinder become very large. This resonant phenomenon is called trapped mode . In this paper, to investigate trapped mode , the diffraction problem of an array of rigidly fixed plates on shallow water in waves is studied. The effects of the number and space of the plates on the resonant frequencies are shown. The relationship among transmitted wave amplitude and the resonant frequencies is also shown. Considering the diffraction problem of two plates in waves, some prediction formula for trapped mode zone are proposed.