The purpose of this paper is to explore the effects of bed temperature and particle size distribution (PSD) on minimum fluidization velocity (U-mf). Quartz sand and bottom ash were fluidized in a bench-scale bubbling fluidized bed reactor at atmospheric pressure with four size distributions including narrow-cut, Gaussian-type, binary and uniformly distributed cases. Bed temperature ranged from 30 degrees C to 600 degrees C. The results show that the Umf of particles with wide PSD decreases with the increase in bed temperature and varies with the mass fraction of coarse particles due to different interparticle forces. In comparison with the narrow-cut particles with the same average diameter, a significant difference was found, which indicates that conventional equations predicting Umf of narrow-cut particles are not appropriate for wide size distributed particles. Due to apparent "filling" and "lubricating" effects, binary and uniformly distributed particles exhibit lower Umf than that of Gaussian-type particle distribution. The semi-fluidized region during fluidization of wide size distributed particles extends with increasing temperature and becomes stable at higher temperature region. To precisely calculate the Umf of wide-range sized particles, a more accurate and simpler correlation was proposed based on the experimental data. (c) 2012 Elsevier B.V. All rights reserved.