Seed particles were coated with a powder in a fluidized bed by atomizing a suspension of the powder and then measuring the coating efficiency. This depends on the operating conditions, for example, the concentration of the powder and binder in the suspension, the bed temperature, the humidity of fluidizing gas and the feed rate of the suspension. An index, R, which evaluates quantitatively the drying conditions in a fluidized bed, is defined as the ratio of the humidity of outlet gas to its maximum humidity achieved at the saturation. It was confirmed that the effects of bed temperature, humidity of fluidizing gas and feed rate of suspension on the coating efficiency were all correlated by the index. The index proposed here is useful for estimating the coating ability of the materials used and of the fluidized bed coater.