The present invention relates to a flexible optical waveguide including a core portion and a cladding layer which has an increment of insertion loss of 0.1 dB or less upon subjecting the waveguide to a 360.degree. bending test at a radius of curvature of 2 mm, as well as an optical module using the flexible optical waveguide. There are provided a flexible optical waveguide having high flexing property, heat resistance and transparency, as well as an optical module using the flexible optical waveguide.