Anaerobic digestion (AD) of lignocellulosic biomass has received significant attention for bioenergy production in recent years. However, hydrolysis is a rate-limiting in AD of such feedstock. In this study, effects of hydrothermal pretreatment of Napier grass, a model lignocellulosic biomass, on methane yield were examined through series of batch and semi-continuous studies. In batch studies, the highest methane yield of 248.2 +/- 5.5 NmL CH4/g volatile solids (VS)(added) was obtained from the biomass pretreated at 175 degrees C, which was 35% higher than that from the unpretreated biomass. The biomass pretreated at 200 degrees C resulted in formation of 5-hydroxymethylfurfural and furfural, which significantly inhibited methanogenesis. In semi-continuous studies, digester fed with the biomass pretreated at 200 degrees C at organic loading rate (OLR) of 4 g VS/L.d resulted in digester failure. Thus, OLRsoluble/OLRtotal ratio < 200 is proposed as an operating criterion for effective operation of digester fed with pretreated biomass slurry.