Disclosed is an optical fiber suitable for WDM system, particularly whose zero-dispersion wavelength is positioned in a short wavelength band less than 1,300 mm. In the optical fiber, dispersion has a positive value, not zero, at 1,310 nm, and a dispersion slope is positive at 1,550 nm with dispersion of 25 ps/nm-km or less. In addition, an effective sectional area is 65 .mu.m.sup.2 or less at 1,310 nm, and 80 .mu.m.sup.2 or less at 1,550 nm.Thus, though a transmission signal is Raman-amplified at a wavelength band of 1,300.about.1,700 nm, transmission characteristics are not deteriorated due to crosstalk between pump signals. In addition, since the optical fiber has smaller effective sectional area than a general single-mode optical fiber with having substantially the same dispersion feature, it gives better Raman gain efficiency than a general single-mode optical fiber.