The present invention relates to an optical transmission line comprising a structure for effectively lowering both of nonlinearity and dispersion slope, and an optical transmission system including the same. The optical transmission line comprises, as a repeating transmission line disposed between stations, a single-mode optical fiber having a zero-dispersion wavelength in a 1.3-.mu.m wavelength band and a dispersion-compensating optical fiber for compensating for the chromatic dispersion of the single-mode optical fiber. The optical transmission line has an average dispersion slope S.sub.ave of -0.0113 ps/nm.sup.2 /km or more but 0.0256 ps/nm.sup.2 /km or less at a wavelength of 1550 nm, and an equivalent effective area EA.sub.eff of 50 .mu.m.sup.2 or more at the wavelength of 1550 nm, whereas the average dispersion slope S.sub.ave and the equivalent effective area EA.sub.eff are designed so as to satisfy a predetermined condition such that the bending loss falls within a permissible range of 2 dB/m or more but 10 dB/m or less.