Fluidization of non-spherical particles commonly exists in the biomass utilization and municipal solid waste (MSW) processing industries. In this study, cylindrical particles are used as a typical type of non-spherical particle and its orientation distribution is investigated when being co-fluidized with small spherical bed material. X-ray particle tracking velocimetry (XPTV), based on an X-ray stereography imaging system, is used to measure the 3D orientation of a single tracer particle over a long time period in the fluidized bed. The effects of gas velocity (u(f)), cylindrical particle mass fraction (omega), particle sphericity (Phi), and bed material size on the orientation distribution of the cylindrical particle are investigated and discussed in detail. An orientation distribution probability density function (PDF) model is proposed based on all experimental results. The distribution probability P of the angle between the cylindrical particle central axis and vertical direction lambda across the bed shows two minima in the ranges 0 degrees <= lambda< 10 degrees and 40 degrees <= lambda < 50 degrees, and two maxima in the ranges 20 degrees <= lambda < 30 degrees and 70 degrees <= lambda < 80 degrees. Increasing u(f) reduces lambda, while the effect of particle sphericity Phi and cylindrical particle mass fraction omega increases with increasing u(f). (C) 2019 Elsevier Ltd. All rights reserved.