The hemicellulose and cellulose were extracted with 24% KOH and 2% boric acid at 20oC for 2 h from pressure refined wheat straw holocellulose. The yields were 26.2-28.4% and 41.8-43.0%, respectively. The hemicellulosic material was further fractionated into hemicellulosic type A-C. The contents of neutral sugars, uronic acids and molecular-average weights of hemicellulosic fractions B were measured. Xylose was the major sugar in the hemicellulosic fractions, while galactose and glucose were the minor constituents and arabinose appeared in noticeable amounts. The contents of phenolic acids and aldehydes, obtained by alkaline nitrobenzene oxidation, in hemicellulose B were 0.33 (>300 ?m TMP fiber) and 0.49% (=<300 ?m TMP fiber), respectively. The methylation analysis and 13C NMR measurement showed that hemicellulose B, isolated from pressure refined wheat straw (>300 ?m TMP fiber), appeared to be essentially a (1->4) linked ?-d-xylan with 4-O-methyl-?-d-glucopyranosyluronic acid attached at position 2, while l-arabinofuranosyl and d-xylopyranosyl groups attached at position 3.