A new method of wet state (WS) sodium hydroxide (NaOH) was advanced to pretreat corn stover for enhancing biogas production. The results showed that 88% moisture content, 3-day treatment time and ambient temperature (20,C) was appropriate for WS NaOH pretreatment. The NaOH dose of 2% and the loading rate of 65 g/L were found to be optimal in terms of 72.9% more total biogas production, 73.4% more methane yield, and 34.6% shorter technical digestion time, as compared to the untreated one. WS pretreatment used 86% shorter treatment time and 66.7% less NaOH dose than solid state one. The analyses of chemical compositions and chemical structures showed that 9.3-19.1% reduction of the contents of total Lignin, cellulose, and hemicellulose (LCH), and 27.1-77.1% increase of hot-water extractives contributed to the enhancement of biogas production. WS NaOH pretreatment could be one of cost-effective methods for high efficient biological conversion of corn stover into bioenergy. (C) 2009 Elsevier Ltd. All rights reserved.