Although radiation-chemical reactions have been applied to many industrial fields, especially to polymer industries, they are mainly aimed at improving mechanical, thermal and surface properties of the materials. Application of radiation chemical reactions to the construction of functional organic materials, such as electronic and optical materials, is relatively few. However, radiation-chemical reactions have the following characteristics that are advantageous for studying and constructing functional organic materials.(1) The reactions take place even in solid without structural deformation, which is advantageous for modifying chemical and physical properties of precisely shaped solid substances.(2) The reactions take place within the region where the radiation energy is absorbed, so that only a part of a solid substance can be modified with precise spatial resolution. Moreover, the region is controllable three dimensionally by changing the field of exposure and the depth of penetration that depends on energy of the ionizing particles.(3) The reactions can be induced without additives, which is advantageous for maintaining purity of the materials.We would like to show how radiation chemical reactions are useful for the construction of functional organic materials by taking our recent studies on resist polymers for radiation lithography and carbon nano-fibers that are considered to be functional materials for next generation lithography and electronics, respectively.The resist polymers synthesized were polyethylene and poly (methyl acrylate) with benzyl ester of carboxylic acid at the center of the polymer skeleton. Introduction of the benzyl ester to the polymer skeletons changed the polystyrene and the poly (methyl acrylate) from cross-link type to scission type polymers upon γ irradiation.The irradiation of the polymers resulted in binary change of the molecular weight, due to the dissociative capture of secondary electrons by the benzyl ester, asMnR1COOCH(C6H5)R2Mn + e- → MnRiCOO- + CH(C6H5)R2MnThe selective scission at the center doubled the sensitivity to ionizing radiations, since random scission necessitates two scissions per skeleton for reducing the averaged molecular weight to a half. The generated polymer fragments were not decomposed by further irradiation, which suggests that the synthesized polymers have high resistivity to plasma etching.Amorphous carbon nano-fibers with a uniform diameter of an order of 10 to 100nm and high electron conductivity were produced by ion-beam irradiation of decacyclene whiskers that were generated during the vapor deposition. A fluence of about 1016/cm2 was necessary for converting the whiskers to the carbon nano-fivers with 200 keV N+ ion beams. This novel method of preparation is advantages for using the fibers as a new functional material such as a support for the catalyst of fuel cells, since the spatial characteristics of the fibers are easily controllable.