A model for estimating the agglomerate size of Geldart group C powder in a vibro-fluidized bed was proposed. The agglomerate size was estimated by using the experimental value of the lower limit of gas velocity for channel breakage, U-cha, and the void fraction, E-cha, at U-cha. The force balance between cohesive force (van der Waals force) and separation force (gravity, vibration and shear force by a gas now) was considered. The vibrational force was considered to act on the agglomerates to increase the effect of gravitational force. The calculated agglomerate size decreased with increasing vibration strength. The value of U-cha decreased with increasing vibration strength for all tested group C powders in this study. The effects of particle properties such as particle diameter, density and material on u(cha) were discussed. It was found that a higher value of u(cha) was obtained when the cohesive force was large in the cohesive-separation force balance. This indicates the particle properties affect the cohesive-separation force balance.