Anaerobic digestion is the most cost effective technology for sustainable biogas production from rice straw. Rice straw was subjected to ambient pretreatment with alkali and subsequently digested anaerobically. A dimensional equation was developed to predict the efficacy of alkali treatment in terms of soluble COD. Biomethanation process parameters like temperature, initial pH, particle size, substrate/inoculum ratio, trace element supplementation, C/N ratio and hydraulic retention time were optimized. The highest biogas production under optimized conditions was 514 L/kg VS/day (similar to 59% CH4) from milled rice straw (1 mm) pretreated with sodium hydroxide (1% w/v) at ambient temperature for 180 min. The digester was operated at 15 days HRT at 37 degrees C and neutral pH. C/N ratio was optimized at 25 using urea. Higher biogas yield from rice straw treated with lower concentration of NaOH at ambient temperature may make this process more economical than the previous reports. (C) 2016 Elsevier Ltd. All rights reserved.