A planar waveguide was formed in Er:NaY(WO4)2 crystal by the implantation of 3.0 MeV Si+ ions at a dose of 2 × 1015 ions/cm2 at room temperature. The prism-coupling method was used to observe the dark modes of the waveguide. Both TE- and TM-polarized light were carried out to measure the effective refractive index of the modes. Refractivity calculation method (RCM) was used to reconstruct the profiles of the refractive index in the waveguide. Moreover, the lattice disorder and the ionization damage of 3.0 MeV Si in Er:NaY(WO4)2 were obtained by using the simulation of TRIM'98 (transport of ions in matter) code. © 2002 Elsevier Science B.V. All rights reserved.