Optical channel waveguides in Nd/Ce codoped YAG laser crystal have been fabricated by using mask-assisted carbon ion implantation. The measured waveguide mode distributions were in good agreement with the calculated modal profiles, which implies the feasibility of designable devices. After thermal annealing treatment at $260°\mathrm{C}$ for $30\min$ in air, the propagation loss of the waveguide was reduced down to $\sim 2\mathrm{dB}/\mathrm{cm}$ at a wavelength of $632.8\mathrm{nm}$ . The microluminescence spectra of the waveguides show that the fluorescence properties of both Ce and Nd ions (including the energy transfer between them) are not significantly affected by the waveguide formation processing, which indicates a fairly good potential for further laser actions in a compact device.