Radiation-induced attenuation (RIA) in undoped-silica-core optical fibers, of which the preform cores are synthesized in high O 2 excess conditions in the vapor-gas mixture ("high oxygen excess fibers, HOE-fibers"), is investigated in the spectral range 1.1-1.7 μm under γ-irradiation from a 60 Co-source. The O 2 excess degree, the F content in the cladding, the drawing speed, and tension are varied among the fibers. RIA is found to strongly depend on the balance between the O 2 excess degree in the core and the F content in the cladding and also on the fiber drawing tension. RIA anticipated in HOE-fibers with optimized O 2 excess and a practicably low drawing tension at λ = 1.55 μm is estimated to be a few times lower than that in the commercial radiation-resistant F-doped-silica-core fibers.