An optical fiber includes a core and a cladding surrounding the core. An absolute value of dispersion at a wavelength of 1550 nanometers is equal to or more than 4 ps/nm/km and equal to or less than 10 ps/nm/km. An absolute value of dispersion slope at the wavelength of 1550 nanometers is equal to or less than 0.04 ps/nm.sup.2/km. An effective area at the wavelength of 1550 nanometers is equal to or more than 40 .mu.m.sup.2. A transmission loss at the wavelength of 1500 nanometers is equal to or less than 0.205 dB/km.