Effect of ultrasound (US) on cellulose decomposition was studied in the solution and hydrogels, when the US with different frequencies and powers were exposed to the cellulose having different molecular weight. Gel permeation chromatography (GPC) results showed that higher US frequency and power were much more effective to depolymerize cellulose. The cellulose in looser network hydrogel and lower cellulose concentration solution were easier to be depolymerized by US irradiation. The effect of US exposure was more effective on the cellulose with lower molecular weight in both hydrogel and solution configuration. Moreover, the results also showed that US could depolymerize the cellulose more effectively in solution than hydrogel. Viscoelasticity showed that the gel network was broken by the US exposure.