Results of investigations of a granulated bed dynamics during agglomeration in rotating drums were discussed. The investigations were carried out in granulators with continuous wetting of the bed with water, thus making no difference whether the wetting time or bed moisture content was assumed as an independent variable. The variable parameters were: drum diameter D ranging from 0.25 to 0.4 m, drum filling with raw material k in the range from 5 to 20% of inner drum volume and rotational speed of the drum n = 10 to 32 rpm. During granulation the torque value was measured on the granulator shaft. Every minute a sample was taken from the drum, the angle of natural repose of the product, and its bulk density were determined. A dimensionless equation describing the bed dynamics during granulation process was proposed.