NaOH and its derivatives are used as pulping reagents, wherein the spent NaOH is recovered in salt form and reused. In this study, low concentration of NaOH (approx. 1-1.5%) was used for pretreatment of corn stover and hybrid poplar (high lignin poplar and low lignin poplar). It was done with the understanding that NaOH can be recovered. In alkaline pretreatments, it is technically feasible to retain most of carbohydrates while removing large fraction of lignin in the biomass. The enhancement of digestibility is caused primarily by lignin removal. Retention of hemicellulose after pretreatment gives a significant economic benefit since it eliminates the need of detoxifying hemicellulose sugars. The main objective of this study is to see if NaOH treatment can achieve that goal. For this purpose, wide range of pretreatment conditions (temperature, NaOH concentration, and reaction time) were applied and the composition and the digestibilities of the treated biomass were examined. From the results, the optimum process conditions were identified for each of the feedstocks on the basis of carbohydrate retention and digestibility of treated solid. Among the noteworthy findings was that corn stover, after treated with NaOH under moderate conditions, attains near 100% glucan digestibility. On the other hand, hybrid poplar required treatment with much higher severity, in terms of temperature and NaOH concentration, in order to attain acceptable digestibility. Modification of the pretreatment was also attempted including addition of hydrogen peroxide during NaOH treatment.