The critical transition velocity, U-cr, previously defined by Liang et al. [W.-G. Liang, S.-L. Zhang, J.-X. Zhu, Y. Jin. Z.-Q. Yu and Z.-W. Wang, Flow characteristics of the liquid-solid circulating fluidized bed, Powder Technol., 90 (1997) 95-102.] to demarcate the liquid-solid conventional and circulating fluidization regimes, was found to vary with the total solids inventory and the solids feeding system. In this work, an onset velocity for circulating fluidization regime, U-cf, is proposed to give the lowest U-cr value and to provide a convenient demarcation velocity that is independent of system geometry. This liquid velocity is obtained by measuring the time required to empty all particles in a batch operated fluidized bed under different liquid velocities. This method can be used for a wide range of particles and involves less influence of the operating conditions such as the solids inventory and the solids feeding system. Compared to the critical transition velocity, this newly defined onset velocity is a more intrinsic parameter, only dependent on the liquid and particle properties. Based on the experimental results obtained in this work and other published results, the influence of particle properties and equipment setup on the onset velocity is also discussed.