A dispersion-compensating fiber having a figure of merit of at least 200 ps/nm/dB and an absolute value of relative dispersion curvature of at most 10.sup.-4nm.sup.-2 both at a wavelength of 1550 nm, a dispersion compensator, and an optical transmission line are provided for large-capacity optical transmission. The dispersion-compensating fiber has a first core region with a refractive index of n.sub.1, a second core region surrounding the first core region with a n.sub.2 (n.sub.1>n.sub.2), a third core region surrounding the second core region with a n.sub.3 (n.sub.2n.sub.4). With reference to n.sub.4, the first core region has a relative refractive-index difference of 2.0% to 3.3%, the second core region of -1.5% to -0.2%, and the third core region of 0.2% to 0.5%.