We have studied the effect of nonuniformity in graded index, channel waveguide dimensions on the phase‐matching condition for guided‐wave nonlinear frequency conversion. Both theoretical and experimental results using normalized waveguide parameters and quasi‐phase matching for second‐ harmonic generation in annealed proton‐exchanged LiNbO3waveguides show the existence of an optimum waveguide design which is insensitive to inhomogeneities in the waveguide dimensions. Application of such a waveguide design using normalized approach can significantly relax the fabrication tolerance, leading to nonlinear guided‐wave devices with long interaction lengths and useful conversion efficiencies.