An optical fiber, which is employed as a transmission line of an optical communication system, has a cable-cutoff wavelength of not longer than 1430 nanometers at the wavelength of 1450 nanometers, a mode-field diameter of not less than 7 micrometers and not more than 9 micrometers, a transmission loss of not more than 0.285 dB/km, and a dispersion of 0.1 to 4 ps/nm/km.