N-methylmorpholine-N-oxide has shown a positive effect for the pretreatment of lignocelluloses. Pretreatment by NMMO was developed to enhance the digestibility of lignocellulosic biomass. Barely straw and forest residue were pretreated by N-methylmorpholine-Noxide (NMMO) prior to anaerobic digestion. The effectiveness of NMMOtreatment on straw and forest residue was examined as well as the recycling and reuse of NMMO for the next pretreatment process. During the first experimental series pretreatments were performed at 90 °C for 3h and 30h, followed by digestion of the pretreated material for 41 days. Low methane yield was found in these experiments due to high organic loading rate. In the second series the recycling and reuse of NMMO was investigated on straw. The pretreatments were carried out at 90 °C for 30 hr and the recycling and reuse were performed in three times. After treatments with fresh, as well as 1, 2, 3 times recycled NMMO methane yield of 0.45, 0.42, 0.38 and 0.4 Nm3/kg VS were obtained.