The present work focuses on the axial and radial distribution of the solids holdup in the flow development region of a CFB riser. The effects of operating conditions (superficial gas velocity and solids circulation rate) and particle properties (particle density and size) on solids holdup are investigated by an optical fiber probe (PC6M). The solids holdup is higher in the lower section than that in the upper section of the riser, and is lower in the center than in the wall region of the riser. Increasing the solids circulation rate or decreasing the gas velocity results in the increment of the solids concentration and radial non-uniformity. With the increase of the particle density and size, the solids concentration increases, which is more evident at the lower part of the riser. Empirical correlations for predicting the cross-sectional averaged solids holdup and local solids holdup are developed respectively based on lots of experimental data. A good agreement is obtained between the predicted results and experimental data of the present work and many other data reported in the literature. (C) 2018 Elsevier B.V. All rights reserved.