The present study is concerned with microstructural analyses of the surface layer of a high speed steel (HSS) roll modified by irradiation of accelerated electron beam. HSS roll specimens were irradiated using an electron accelerator, and the microstructures of the irradiated surface layers were examined. Upon irradiation, the irradiated surface layer was melted because of the large heat input, resulting in microstructural change from coarse cell structure of primary carbides to a mixture of fine carbide cell and retained austenite. This microstructural modification might improve fracture or fatigue properties of the irradiated surface layer, although its hardness was decreased and quench cracks were found occasionally in the heat affected region. In order to solve the problems for applying the accelerated electron beam technique to HSS rolls, appropriate processing methods such as pre-or post-irradiation were suggested.