As a result of increasing demand for alternatives to fossil energy, the agricultural biogas sector is in expansion and lignocellulosic biomass (LCB) represents an interesting renewable feedstock. Nevertheless, due to the recalcitrance and complexity of its structure, deconstructive pretreatments are necessary to render possible biochemical conversions and efficient biomass exploitation. In this work, chemical-free, mild microwave pretreatment was evaluated (through BMP tests) as a method to improve anaerobic biodegradability of two grass biomass of industrial relevance and contrasted parietal content: corn stalks (low parietal content, high soluble content) and miscanthus (high parietal content, low soluble content). BMP tests carried out on raw biomass before pretreatment highlighted the negative correlation of BMP value to lignin and cellulose contents and the positive correlation to soluble and hemicellulose contents.Efficiency of microwave pretreatment under two conditions, open vessel and high pressure (4 bars), with water as unique solvent was tested for tackling recalcitrance and results were compared to conventional heating pretreatment and a control treatment. Solid and liquid phases were separated after pretreatment with the aim to elucidate if microwave treatment had an impact on organic matter solubilisation and/or on the residual solid phase, which could improve the biodegradability of the pretreated solid fraction. To the authors' knowledge, this is the first study to dissociate methane production of the solid phase from that of the liquid phase after microwave pretreatment.Observed BMP values in mesophilic conditions of raw biomass samples were 286 NLCH4/kgVS for corn stalks and 228 NLCH4/kgVS for miscanthus respectively (in agreement with literature). No significant improvement in BMP value nor in CH4 production kinetics were observed following microwave pretreatment, while a harsh chemical pretreatment (10 h soaking in 10% w/w NaOH) allowed 30% increase in BMP value. These results highlight the significant chemical effect -compared to thermal- on the biomass deconstruction and fibers breakdown during chemical-free microwave pretreatment. A synergy microwave-NaOH effect could allow to achieve higher impact on biomass recalcitrance using lower NaOH amounts than chemical treatment alone.