With two different kinds of particles and three different circulating fluidized (CFB) bed risers, the riser diameter influences on bed density for different particles were investigated by measuring the total pressure drop across the whole riser and the axial profile of apparent voidage. The risers had the same bed height of 3.0 m but different inner diameters of 66, 97 and 150 mm, respectively. Particles of FCC and silica sand were used, which belong to types A and B of Geldart classification, respectively. It was found that riser diameter has opposite influences for Geldart A and B particles on the total pressure drop across the whole riser and the differential pressure gradient (apparent solids concentration) at a specified bed elevation. With increasing riser diameter, these two pressure drops increase for group B but decrease for group A particles, consistent with several other literature measurements in pilot-scale risers. As a result of these different dependencies of bed density on riser diameter, the saturation-carrying capacity, i.e., the solids circulation rate at the dilute suspension collapse, was also found to vary differently with riser diameter for different particles. That is, the increase in riser diameter increases the saturation-carrying capacity for Geldart A but decreases it for Geldart B particles.