Granular material motion in a vibrating bed was investigated using an optical signal analyzing method. When the vessel relative acceleration was greater than some threshold acceleration and the particle was located higher than some threshold height in the bed, the particle vertical vibrating frequency deviated from the vibrator frequency and the particle vibrating frequency was half of the vibrator frequency. When 3.5-mm rubber particles were used, the particle vertical vibrating frequency deviated from the vibrator vibrating frequency when the relative acceleration was greater than 1.75 g and the deviation was observed when there was at least one particle layer below the insert card. The threshold relative acceleration causing the state transition increases with the increase of vibrator frequency. When particles are subjected to vertical vibration, the fluid-like behavior of the particle bed initiated from the surface of the bed with increasing relative acceleration. The interaction of the rising and falling momentum fluxes provides a possible mechanism for the state transition of the granular materials in a vibrating bed. (c) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008