Abstract: Systematic experiments were conducted with FCC particles in a 16m high CFB riser to investigate the solids holdup in a fully developed section of the riser,with superficial gas velocity U _g from 2.0 to 10.3m·s ^-1 and solids circulating rate G _s from 5.4 to 230kg·m ^-2 ·s ^-1 .Of 134 sets of operation conditions employed in the experiments,there were 113 sets of operation conditions under which the gas-solids two-phase flow became fully developed in the upper or even the whole section of the riser.It is found that in a fully developed section of the riser,the average solids holdup ε _s increases linearly with increasing solids circulating rate G _s and decreases with increasing superficial gas velocity U _g,with a trend rather different from that reported in literature.The correlation developed in the paper,for solids holdup in a fully developed section of CFB riser,is in good agreement with the experimental data and also explains the differences among the predicted results of other authors,delineating more clearly the variations of solids holdup in a fully developed section with operation parameters.