The changes in molecular size distribution of rice starch during extrusion cooking and simple heating of rice flour were compared and the effect of subsequent enzyme treatment on the molecular size was examined. A single-screw extruder was used with varing feed moisture contents ($17{sim}29%$) and barrel temperatures ($100{sim}150^{circ}C$). An aluminium capsule immersed in oil bath ($100{sim}200^{circ}C$) was used for the simple heat treatment of rice flour. In case of extrusion cooking the mechanical energy input varied sharply at around 23% moisture content of the feed. At the feed moisture content of $17{sim}23%$, a significant molecular size reduction of rice starch was observed by the gel permeation chromatography using Sephacryl S-1000 gel. The intact starch molecules of above $4{ imes}10^{7}$ dalton were largely disintergrated by extrusion cooking of rice flour containing the moisture content less than 23%. It was mostly degraded further into the molecules having below $5{ imes}10^{6}$ dalton by ${alpha}-amylase$ treatment. But at the feed moisture content above 26% the starch did not show molecular size reduction either by extrusion cooking or by subsequent enzyme treatment. On the contrary little changes in molecular size of starch was occured by simple heating of rice flour containing the moisture less than 20%. but slight size reduction was observed at the moisture content above 23%, where the effect of ${alpha}-amylase$ was also observed.