A radiation resistant single-mode optical fiber has a core and a cladding, each made of fluorine-doped silica glass, in which a chlorine concentration of the core is at least 0.01 ppm, a relative refractive index difference of the core based on the refractive index for silica is between -0.30% and -0.10%, a relative refractive index difference of the core based on the refractive index for the cladding is between 0.3% and 0.5%, a cutoff wavelength is 1.27 .mu.m or below, and a bending loss at a wavelength of 1.3 .mu.m and a bending diameter of 20 mm is 0.5 dB/m or less.