We report phase‐matched second‐harmonic generation in KNbO3 channel waveguides. The guides were fabricated by one single homogeneous He+ ion irradiation and specially structured photoresist on the KNbO3 crystals serving as an implantation mask. The mask enabled the formation of channel guides with a trapezoidal‐shaped cross section providing simultaneous confinement of both TE and TM modes. 2.6 mW second‐harmonic blue‐light at 441 nm was generated in a 5.8‐mm‐long guide for an incident fundamental power of 280 mW. © 1996 American Institute of Physics.