An optically polished x-cut KTiOPO4 crystal of size 22×6×1.5mm3 was implanted with 2.8-MeV He ions to a dose of 1.5 × 1016 ions/cm2 at room temperature to form a waveguide. The prism coupling method was used to measure the modes and the fiber probe technique was used to measure the attenuation in the KTiOPO4 waveguide. The refractive index profile, nz, in the KTiOPO4 waveguide was given based on the procedure by Chandler and Lama [P.J. Chandler and F.L. Lama, Optica Acta 33, 123 (1986)]. The waveguide attenuation measured was 2.57 dB/cm for m = 1 mode. After annealing at 260 °C for 30 min, there was no obvious change in the KTiOPO4 waveguide attenuation.