► Thermo-chemical pre-treatment enhanced the solubilisation of grass silage. ► Pre-treatment caused the highest reduction of the lignin content, followed by reduction of hemicellulose and cellulose. ► Grass silage pre-treated at NaOH loading rates of 5% and 7.5% had the highest specific CH4 yields. ► Anaerobic digestion of 7.5% NaOH pre-treated GS had the longest lag phase. ► 100°C and the NaOH loading rate of 5% is a proper GS pre-treatment condition. Dried grass silage (GS) was pre-treated at different NaOH loading rates (1%, 2.5%, 5% and 7.5% by volatile solids (VS) mass in grass silage) and temperatures (20°C, 60°C, 100°C and 150°C) to determine effects on its bio-degradability in terms of the hydrolysis yield and degradation of ligno-cellulosic materials for biogas production. At 100°C and the four NaOH loadings, up to 45% of the total COD was solubilised and up to 65.6%, 36.1% and 21.2% of lignin, hemicellulose and cellulose were removed, respectively; biological methane production potentials obtained were 359.5, 401.8, 449.5 and 452.5ml CH4/g VS added, respectively, being improved by 10–38.9% in comparison with untreated GS. VS removals following anaerobic digestion were 67.6%, 76.9%, 85.3%, 95.2% and 96.7% for untreated GS and GS treated at the four NaOH loadings, respectively. 100°C and the NaOH loading rate of 5% is recommended as a proper GS pre-treatment condition.