A Gdx Y1-xCa4O(BO3)3 (Gdx Y1-xCOB) crystal, which belongs to rare-earth calcium oxyborate family, is used for non-critical phase-matching of visible and ultraviolet (UV) light.We employ 2.8-MeV He+ ions to form optical waveguides in the Gd0.275 Y0.725COB crystal at room temperature.The doses are varied from 1.5 × 1016 ions/cm2 to 5 × 1016 ions/cm2.A model 2010 prism coupler is used to measure the modes in Gd0.275 Y0.725 COB waveguides formed by MeV He+ ion implantation.The refractive index profile is fitted based on the reflectivity calculation method.The shape of refractive index profile is compared with one of nuclear energy loss obtained by TRIM98 simulation.