In France, wheat straw represents an important part of the crop residues which could be used as renewable energy source biomass. This publication aims at studying the impact of an ultra-fine shredding, named micronisation, of straw on its biodegradability under mesophilic anaerobic conditions. For that purpose, biological methane potential (BMP) tests were performed on various fractions of wheat straw resulting from successive shredding processes. The first one results from a coarse cutting mill, the following were obtained thanks to centrifugal milling. The fractions vary between 804 and 45μm of mean diameter. All results are discussed both in terms of maximal methane potential and in terms of kinetic rates. The main conclusion is that shredding does not improve methane potential of straw but have a significant influence on the biodegradation kinetics. Specific analytical techniques (Infra red spectroscopy, water distribution analysis...) based on biochemical and physical parameters were performed to characterize the fractions and explain these results.