In the present study, the effects of wall roughness on erosion rate for gas-solid flows in horizontal annular pipes for different ratios of inner to outer radii and also for different values of solid particle concentration are investigated. The results are compared with earlier studies, and the effect of various parameters is discussed. The impingement angles and impact velocities of solid particles, which are needed for using the comprehensive erosion model, are evaluated using a four-way coupled Eulerian-Lagrangian simulation approach of gas-solid flows. The turbulent gas phase flow is solved using the k-epsilon model. For the simulation of particle movement, the fluctuations of gas phase velocity are evaluated by the eddy interaction method. The numerical results indicate that the solid particle induced erosion rate at the outer wall of annular pipes is considerably larger in comparison with a simple circular pipe with the same outer radius, and the erosion rate further increases as the radius ratio increases. The results also show that the erosion rates at both inner and outer walls increase as wall roughness increases and are much higher compared with those for smooth walls or walls with a small roughness. (C) 2014 Elsevier B.V. All rights reserved.