Granular materials often experience segregation due to the differences of particle size, density and shape. In this work, particle size-induced segregation is investigated for one typical device - conical hopper which is widely used in the storage and processing of granular materials. In particular, the fundamentals governing particle size segregation during the hopper filling process are revealed by the analysis of DEM data such as particle velocities, trajectories, coordination number (CN) and mixing index. The results show that during the central charging process, particles with large velocities easily bounce to the side wall region while particles with small velocities are obstructed by their neighbours and stop moving quickly. Large particles tend to roll along the free surface and accumulate in the side wall region. Particle scale mixing index decreases with the increase of hopper height and the distance from the centre, indicating the effect of charging height and segregation occurrence along the radial direction. Further, the effect of friction coefficients is examined, and a contour plot is drawn, illustrating that a combination of sliding friction coefficient larger than 0.5 and a moderate rolling friction coefficient in a range of 0.01 d(p) to 0.1 d(p) leads to pronounced segregation. The concentration distribution of small particles for various feeding mixtures is analysed, and general trends are observed to demonstrate the segregation extent at different hopper radial locations. (C) 2018 Elsevier B.V. All rights reserved.