Linear polymethyl methacrylate undergoes main-chain scission when exposed to high energy radiation in air at room temperature. Changes in molecular weight were observed by measurements of intrinsic viscosity which give directly the viscosity average molecular weight M,V,. As a rule M,V,=1.9 M,n, for this polymer in which case it is possible to obtain a direct relationship between M,V, and number average molecular weight Mn, this procedure gives information on number of radiation-induced fracture. Because the plot of 1/M,V, against dose is linear when the radiation dose is more than 5×10,5, rad, it may be concluded that radiation-induced fracture occurs at random and they are proportional to the dose. This may account in the yield of main-chain degradation, G,(S) ,=1.0.,Cross-linked long-chain polymethyl methacrylate also undergoes degradation when exposed to radiation in air at room temperature. By using two methods, namely the gel-sol distribution measurement and the average molecular weight between cross-links M,C, measurement, it was found that M,C, of this polymer linearly increasse and the gel fraction reduces with radiation in dose range 5×10,5,~5×10,6, rad. These results show that degree of cross-linkng steadily decreases under radiation. When the radiation dose is more than 1×10,7, rad, the cross-linked polymer becomes linear polymer thoroughly.