Ultrasonic degradation of cellulose with low molecular weight in NaOH and NaOH/urea solutions was investigated at 20 and 500 kHz frequencies by measuring the solution viscosity. The viscosity decreased with sonication time. A small difference in viscosity ratio which is defined as the ratio of specific viscosity of solution after and before sonication was observed for the longer sonication. The degrees of the polymerization were reduced from 230 to 150 for 30 min sonication at 20 kHz and for 120 min sonication at 500 kHz. From XRD measurement, it was estimated that the crystallinity after sonication was the same as that before sonication. The crystallinity structure after sonication corresponded to Cellulose II. The yield of water-soluble components after ultrasonic irradiation was 20 wt% at 20 kHz in NaOH solutions and 30 wt% for 720 min at 500 kHz in NaOH/urea solution. Oligosaccharides and their derivative were detected by the SEC analysis of water-soluble components after sonication.