The impact of an aqueous ammonia pretreatment on the structural properties, delignifiability, and hydrolysability of wheat straw and corn stover was investigated. The results showed that the aqueous ammonia pretreatment had an excellent delignification ability and that the corn stover exhibited higher susceptibility to an aqueous ammonia attack than wheat straw. In total, 35.6% and 70.0% of the lignin in wheat straw and corn stover were removed, respectively, by the aqueous ammonia pretreatment at 75 ºC with 21% ammonia and a solid:liquid ratio of 1:10 for 20 h. Both lignin and polysaccharides in the corn stover exhibited higher susceptibility to aqueous ammonia than those in wheat straw. In addition, the hydrolysability of corn stover was more susceptible to aqueous ammonia than it was to wheat straw. A chemical structure analysis of different substrates showed that the aqueous ammonia pretreatment removed lignin, broke ester bonds between lignin and hemicelluloses, increased the specific surface area and crystallinity index, and, finally, enhanced the hydrolysis yield.