The effects of gas velocity, solid circulation rate, and particle size on the axial pressure profiles and solid holdup in the downer reactor have been determined. The axial gradients increase with increasing solid circulation rate but decrease with increasing gas velocity. However, the effect of particle size on the pressure gradients and solid holdup is insignificant at the given gas velocties and solid circulation rates. In the fully developed flow region(H>2.0m), pressure gradients calculated from a single particle motion are higher than the experimental values at lower gas velocities(<3.0 m/s),whereas they are similar to the experimental values at higher gas velocities(>3.0 m/s),due to the particle agglomeration or cluster formation that was confirmed by the comparison of slip and terminal velocities of a single and cluster particles.