In this study, flow regions in flat bottomed cylindrical gravity-driven hoppers are investigated in terms of velocity and voidage distributions of particles via GPU-based DEM (discrete element method) simulation. For the first time, the hopper flow with steady discharge is described as nine regions which are essential and critical for better understanding of hopper flow. The variation of solid concentration is less than 1% in the transition flow region. The particle acceleration in the converging flow region decreases with increasing the height along hopper. The arch zone, which has the highest particle acceleration, can be divided into a vertical flow region and a centripetal flow region, based on the directions of partide velocities. The area of the vertical flow region and the centripetal flow region decreases with the-decrease of hopper outlet size, so fewer particles above these two regions flow acceleratedly to the outlet, resulting in the height reduction of the converging flow region and the transition flow region. (C) 2016 Elsevier B.V. All rights reserved.