Tetragonal scheelite-like single crystals Er:NaY(WO4)2 (Er:NYW) have been grown along its 〈101〉 direction by using RF-heating Czochralski (CZ) method. A planar optical waveguide in the as-grown Er:NaY(WO4)2 crystal was formed by 2.6 MeV He+ ions implantation with dose of 1.5×1016 ions/cm2 at room temperature. Using the prism coupling technique, the dark modes were measured: four TE modes and five TM modes were observed in the waveguide. Using the reflectivity calculation method (RCM), the refractive index profiles were reconstructed and analyzed before and after the optical waveguide was annealed at 300 °C for 30 min. The energy loss was simulated by the transport of ions in matter (TRIM) code, and the results were investigated in comparison with the refractive index profile in the waveguide. © 2002 Published by Elsevier Science B.V.