This work aims to explore the scale-up effect on the hydrodynamic natures in a pilot-plant gas phase ethylene polymerization fluidized bed reactor (FBR) based on two-dimensional (2D) transient Eulerian model integrating the kinetic theory of granular flow (KTGF). Unlike lab scale FOR, significant differences in the core-annulus structures caused by the scale-up effect are revealed. The core region in pilot-plant scale FBR is much broader, and the distribution is nearly flat for two phases. The annulus structure is thin, and exhibits a very small particle velocity fluctuation. The bed expanding section could reduce the solid velocity and improve the flow patterns of gas and polymer particles. Simulations are also performed to assess the effects of polymer particle size and gas velocity on the hydrodynamics for the non-pelletizing polyethylene process. With the increase of polymer particle diameter (446 mu m to 1338 mu m), the superficial gas velocity should be increased from 0.60 m/s to 0.90 m/s to achieve better steady fluidized state. The results are helpful for understanding how the local mean two phases' velocities and volume fractions vary in pilot-plant reactor for PE production caused by scale-up effect. (C) 2015 Elsevier B.V. All rights reserved.