Sonication behavior of waxy rice starch in water has been investigated in terms of number average molecular weight and molecular weight distribution evaluated by gel permeation chromatography. Observed results on decreases in number average molecular weight indicated the ultrasonic degradation of waxy rice starch. The rate of degradation was accelerated at or above the temperature where gelatinization started. High sonopower also accelerated the degradation rate. After long sonication, the value of number average molecular weight tended to an limiting constant value, and the molecular weight distribution tended to be fairly narrow. Observing degradation curves of other water soluble α-glucans, dextran, and pullulan, in addition, it was found that branched α-glucans have larger limiting molecular size than the unbranched one has, and an universal degradation curve can be obtained by plotting the ratio of the molecular size of the degraded α-glucan to the correspondent limiting one against time.