An optical fiber, made of silica-based glass, comprising a core and a cladding. The optical fiber having a mode field diameter of 6.5 .mu.m or larger at a wavelength of 1300 nm, transmitting light with a wavelength of 1250 nm in a single mode, and having a bending loss of 1 dB/turn or smaller at a wavelength of 1300 nm when the optical fiber is bent with a curvature radius of 1.5 mm.