Rice-straw treated with 5% ammonia water was hydrolyzed with the beta-xylanase of Streptomyces sp. E-86, and four kinds of hetero-oligosaccharides, two arabinoxylo- and two glucuronoxylo-oligosaccharides, were isolated from the hydrolysate by charcoal column, gel filtration, ion-exchange, and aminopropyl silica column chromatographies. The structures of the oligosaccharides were identified as 3(2)-alpha-L-arabinofuranosyl-xylobiose, 3(2)-alpha-L-arabinofuranosylxylotriose, 2(3)-4-O-methyl-alpha-D-glucuronosylxylotriose, and 2(3)-alpha-D-glucuronosylxylotriose by the analysis of component sugar, partial acid hydrolysis, methylation analysis etc. Based on the structures of above oligosaccharides, the structure of rice-straw arabinoglucuronoxylan and the specificity of the xylanase toward the xylan are discussed.