This paper presents a numerical study of the effects of friction and restitution coefficients of particles on horizontal pneumatic conveying by a combined approach of computational fluid dynamics for gas and discrete element method for particles. The flow behaviours are analysed in terms of particle flow pattern, gas pressure drop, solid concentration, particle velocity and transition of flow regime. The results show that in the slug flow regime, an increased friction coefficient or a decreased restitution coefficient results in longer slugs but no change in slug shape. Slug flow cannot be obtained at a low friction coefficient. In the stratified and dispersed flow regimes, the transition between the two regimes and formation of clusters occur due to the changes of friction and/or restitution coefficient. These two coefficients also affect the pressure drop, particle velocity and solid concentration. The resulting effects vary with flow regime. Overall, the effect of restitution coefficient is less significant than that of friction coefficient. It is also found that when gas velocity is varied from high to low values, three flow transition modes may occur depending on friction and restitution coefficients. They are respectively represented by Mode I for dilute-phase only, Mode II for unsmooth transition from dilute-phase to unstablezone, and finally to slug-flow, and Mode III for smooth transition from dilute-phase to slug-flow. Based on the numerical results, a diagram is established to predict these transition modes as a function of friction and restitution coefficients. (C) 2013 Elsevier B.V. All rights reserved.