Polysaccharide fractions were extracted from partially delignified bamboo (Neosinocalamus affinis) culms pretreated with ultrasonic irradiation for varied times and cold sodium hydroxide/urea solution, and their structure and thermal stability were comparatively characterized. In this case, ball-milled bamboo culms were treated with ultrasonic irradiation for varied times (0, 5, 15, and 25 min), dissolved with 7% sodium hydroxide/12% urea solution at -12 degrees C, and then extracted with ethanol and dioxane to obtain partially delignified solid fractions. Subsequently, the solid fractions were subjected to be extracted with dimethyl sulfoxide followed by precipitation in ethanol and yielded the polysaccharide fractions. Sugar analysis indicated that the total sugar content increased from 60.63% in the polysaccharide fraction prepared without ultrasonic irradiation to 81.26% in the polysaccharide fraction prepared with an ultrasonic irradiation time of 25 min. Glucose (similar to 50-55%) was the major sugar component, and xylose (similar to 41-44%) was the second major sugar in polysaccharide fractions in all cases. Spectroscopy (FTIR, (1)H-NMR, (13)C-NMR, and HSQC) analysis suggested that the polysaccharide fractions were mainly composed of (1 -> 4)-linked alpha-D-glucan from amylose and (1 -> 4)-linked beta-D-xylan attached with minor amounts of branched sugars from hemicelluloses. In addition, thermal analysis showed that the main degradation stage of the polysaccharide fractions occurred between 210 and 320 degrees C. Compared to the polysaccharide fraction prepared without ultrasonic irradiation, the polysaccharide fraction prepared with ultrasonic irradiation had a slightly lower thermal stability. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 176-185, 2011