Optical fibers to form an optical transmission line suitable for WDM transmission in a wide-spreading wavelength band, having the following characteristics and parameters: a dispersion in absolute value of 0.5 ps/nm/km to 9 ps/nm/km in a wavelength band of 1430 nm to 1625 nm, a dispersion slope in absolute value of 0.04 ps/nm.sup.2/km or less at a wavelength of 1550 nm, a mode field diameter of 7 .mu.m or less at a wavelength of 1550 nm and a cable cutoff wavelength of less than 1430 nm; core 11 surrounded by cladding 7, core 11 being at least two-layered (first layer 1 at the center and second layer 2 surrounding the first layer; relative refractive index of glass layer .DELTA.1 with reference to the cladding being adjusted to not less than 0.6 but not more than 1.6%, relative refractive index of second layer .DELTA.2 with reference to the cladding being adjusted to a negative value.