Provided are a planar optical waveguide and a method of fabricating the same which is adapted to a planar optical component and an optical component for use in a optical communication. The planar optical waveguide includes: a lower cladding layer formed on a substrate, a core formed on the lower cladding layer, a dielectric layer covering the core, and an upper cladding layer formed on the lower cladding layer having the dielectric layer. By forming the dielectric layer having a low refractive index between the core and the clad, a difference of refractive indices between the core and the clad increases so that light is densely focused into the core, thereby forming a single mode having a strong energy to minimize a propagation loss.