A single-polarization optical fiber comprising a core, an inner cladding surrounding the core, an outer cladding surrounding the inner cladding and at least one pair of stress-applying portions each of which are partially extended into both the inner and outer claddings and which are symmetrical about the axis of the core and having thermal expansion coefficient higher than those of both the inner and outer claddings. When the refractive index of the stress-applying portions is lower than that of the core and higher than that of the inner cladding and further not higher than that of the outer cladding, one of orthogonally polarized mode is cutoff and only one polarized mode can propagate in the optical fiber.