Disclosed is an optical fiber suitable for an optical transmission line used in WDM system, particularly a single-mode optical fiber whose zero-dispersion wavelength is positioned in a short wavelength band (less than 1370 mm) so as to enable high-speed mass-storage signal transmission over S-C-L band (1460˜1625 nm) and whose dispersion value and effective sectional area are optimized. In the optical fiber, a dispersion value is at least 9 ps/nm-km at 1460 nm, an effective sectional area is 45˜65 μm2 at 1460 nm, a zero-dispersion wavelength exists at 1370 nm or less, and a dispersion slope is positive. In addition, RDS (Relative Dispersion Slope) is 0.0032˜0.0038 nm−1 at 1550 nm. Thus, the optical fiber enables to repress non-linearity and signal distortion to the maximum during 320 km repeaterless transmission with a transmission rate of 10 Gb/s or more over S-C-L band, a channel spacing of 50 GHz or less, 16 channels, and a signal power of 0 dBm/ch or 2 dBm/ch.