The elutriation of fine particles (group C or A particles) from a fluidized bed of fine-coarse particle mixtures was investigated in a steady state. Alumina particles were used as fine; PVC, alumina, SiC and silica sand particles (group A or B particles) were used as coarse particles. The effect of the relative humidity on the elutriation rate constant was investigated for different elutriated particle sizes and bed particles. There were three different zones between the elutriation rate constant and the relative humidity. At the lower relative humidity, the elutriation rate constant increased with increasing relative humidity. When the relative humidity reached a certain value, the elutriation rate constant was not affected by the relative humidity. We defined this relative humidity as the electrostatic relative humidity (RHe). In the middle relative humidity region, the elutriation rate constant was not affected by the relative humidity. When the relative humidity increased and reached a certain higher value, the elutriation rate constant decreased with an increase of relative humidity. We defined this relative humidity as the capillary relative humidity (RHc). When the relative humidity is smaller than RHe, the elutriation rate constant is affected by the electrostatic forces of the bed particles and the elutriation rate constant increased with an increase of relative humidity. When the relative humidity is higher than RHc, the elutriation rate constant is affected by the capillary forces of the bed particles and the elutriation rate constant decreased with an increase of relative humidity.