Abstract: Based on the earlier works concerning the formula of the change in the axial depth of a bed in a diameter-invariable rotary kiln, the formula of axial depth of bed in a diameter-variable kiln was deduced to adapt to the diameter enlargement at kiln inlet and the turn-down rigs set on the kiln lining. Then, the formulae of mean residence time of solids in both diameter-invariable and diameter-variable kilns were deduced, where several factors were taken into account, such as the inner diameter, total length, discharge dam height, axis inclination and rotation speed of kiln mass feed rate of solids, and the particle diameter and dynamic angle of repose of solids. Cold-state experiments were carried out on a laboratory rotary kiln to verify the formulae proposed, and the results showed that calculated results of the residence time of solids are basically in agreement with the experimental values, with no correction coefficient required because of small errors.