Safflower straw, an abundant, inexpensive, and renewable lignocellulosic waste, was used as a substrate of anaerobic digestion to produce biogas. Hydrothermal pretreatment was carried out on the straw at 120, 150, and 180 degrees C for 1, 2, and 5 h to increase the biogas yield. The pretreatment resulted in a solid (mainly cellulose) and a liquid (mainly hemicellulosic monomers). The highest biomethane yield from solid fraction (191.4 NmL/g VS) was obtained at the least severe pretreatment conditions (120 degrees C for 1 h), which showed 98.3% improvement in comparison to the untreated straw. The maximum methane yield of 406.9 NmLig VS was attained from the liquid fraction of pretreatment at 180 degrees C for 1 h. Overall, at the optimum pretreatment conditions (120 degrees C for 1 h), 148.4 m(3) methane was produced from each ton of pretreated safflower straw, whereas the methane production from the untreated straw was 86.9 m3 methane. Furthermore, enzymatic hydrolysis was carried out on solid fractions. The results revealed that the most severe pretreatment conditions (180 degrees C for 5 h) led to the highest released sugar concentration of 25.1 using the enzyme loading of 10 FPU/g substrate, while it was only 4.5 g/L for the untreated one. (C) 2019 Elsevier Ltd. All rights reserved.