Grinding plant biomass may allow the lignocellulosic assembly to become more reactive/accessible by providing energy for polymer dissociation, increasing contact surface (particle size reduction) and reducing cellulose crystallinity. Moreover lignocellulosic composition varies considerably affecting biomass processability as resource for bio-based energies, composite materials and chemicals. The aims of this work were: (i) to analyse composition of wheat anatomic parts present into wheat straw, (ii) to characterize the behaviour of major components upon successive centrifugal grinding steps (2 mm-screen cutting milling followed by 4-step centrifugal grinding) and (iii) to relate particle size distribution and component concentrations into the finest sized product (0.12 mm-screen ground). The powders from successive centrifugal grindings were sieved and their chemical compositions were determined. Ground straw powders were heterogeneous according to different particle aspects: size, shapes and roughness. In general fractions with lower particle size had higher ash and protein contents whereas cellulose contents are higher in the larger fractions. Wheat straw exhibited a non homogeneous reduction behaviour when finely ground. Fraction compositions were only slightly distinct suggesting that although sieving can constitute a preliminary fractionation step, it is necessary to reduce still more the particle size to reach more effective dissociation of macromolecules assembly. (C) 2010 Elsevier B.V. All rights reserved.