Few studies have investigated scale-up of the residence-time distribution (RID) of particles in bubbling fluidized beds (BFBs) with continuous particle flow. Two approaches were investigated in this study: first, using well-known scaling laws that require changes in particle properties and gas velocity; second, using a simple approach keeping the same particles and gas velocity for different beds. Our theoretical analysis indicates it is possible to obtain similar RTDs in different BFBs with scaling laws if the plug-flow residence time (t(plug)) is changed as m(0.5), where m is the scaling ratio of the bed; however, neither approach can ensure similar RTDs if tplug is kept invariant. To investigate RID variations using two approaches without changing t(plug), we performed experiments in three BFBs. The derivatives dE(theta)/d theta (where E(theta) is the dimensionless RID density function and theta is the dimensionless time) in the early stage of the RTDs always varied with m(-1), which was attributed to the fact that the particle movement in the early stage were mainly subject to dispersion. Using the simple approach, we obtained similar RTDs by separately treating the RTDs in the early and post-stages. This approach guarantees RID similarity and provides basic rules for designing BFBs. (C) 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.