An optical fiber whose value of a ratio of a dispersion to the dispersion slope ranges between 50 nm and 150 nm at the wavelength of 1590 nm. Also, a dispersion compensated optical fiber includes a depressed layer formed of at least one of a plurality of core layers covering the outer circumference of the center core. The depressed layer has -0.7% or less of a relative refractive index difference with the clad. The dispersion compensated optical fiber has the ratio of the dispersion to the dispersion slope falling within a range of between 30 nm and 80 nm at the wavelength of 1550 nm, or a dispersion value not larger than -20 ps/nm/km, a dispersion slope not larger than -0.05 ps/nm.sup.2/km, and an effective core cross sectional area A.sub.eff falling within a range of between 8 and 13 .mu.m.sup.2.