Different biomass types display very different characteristics relating to pelletization conditions and pellet quality. The aim of this study is to analyze the effect of biomass composition and milling size on pelletization parameters and the physical quality of the pellets obtained. Nine different woody and non woody biomass raw materials were pelletized in a bench scale pellet press. Materials were milled at 2 mm and 4 mm screen size and compositional analysis of all the tested biomasses was performed. Pellet mechanical durability and fines content were analyzed as well as the power demand of the press and temperature of the die reached during pelletization. In addition, scanning electron microscope images of the raw materials and the transverse fracture of the pellets were obtained. Pellets obtained from 2 mm milled materials showed higher mechanical durability values. Pellets obtained from woody materials, which exhibited lower extractives and higher lignin contents, evidenced also a higher physical quality. Power demand and temperature of the die were also higher for woody materials, suggesting a higher friction inside the die channels. SEM images showed a closer agglomeration of the particles when materials were milled at 2 mm. Woody particles had a more evident deformation when pelletized while herbaceous materials preserved their shape. Different bonding interactions could be observed in pellets from different materials. (C) 2014 Elsevier Ltd. All rights reserved.