Disclosed is a single-mode optical fiber suitable for and optical transmission line used in WDM (Wavelength Division Multiplexing) system, which has low dispersion slope, sufficient dispersion value and large effective section area over S-, C- and L-band (1460.about.1625 nm) to enable high-speed, large-capacity signal transmission. The optical fiber uses the wavelength region from 1460 to 1625 nm, and the optical fiber also has a dispersion value of 0.1.about.3.0 ps/nm-km, more preferably 0.3.about.2.4 ps/nm-km, at 1460 nm, a dispersion value of 3.0.about.5.5 ps/nm-km, more preferably 3.2.about.5.2 ps/nm-km, at 1550 nm, and a dispersion value of 4.5.about.8.0 ps/nm-km, more preferably 4.8.about.7.7 ps/nm-km, at 1625 nm. In addition, the optical fiber has a dispersion slope of 0.023.about.0.05 ps/nm-km.sup.2 at 1550 nm, an effective sectional area of 35.about.50 .mu.m.sup.2 at 1550 nm, an effective section area of 35.about.50 .mu.m.sup.2 at 1460 nm. Thus, though the signal is transmitted through S-, C- and L-band, this optical fiber may suppress nonlinear phenomenon and signal distortion to the minimum.