We demonstrate a 1.55 μm GaAs/GaNAsSb/GaAs optical waveguide grown by molecular beam epitaxy as an alternative to the AlGaAs/GaAs system. The 0.4-μm-thick GaNAsSb guiding layer contains ∼3.5% of N and 9% of Sb, resulting in optical band gap of 0.88 eV. The refractive index of the GaNAsSb layer was measured from 800 to 1700 nm. The GaNAsSb layer has a refractive index value of 3.42 at 1.55 μm wavelength. The propagation loss measured using the Fabry–Pérot resonance method was found to be affected by nitrogen-related defect absorption.