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We explore an efficient method to couple light from a dielectric medium into a two-dimensional photonic-crystal waveguide (PCW). This is achieved by inserting a graded-index photonic crystal (PC) between a dielectric waveguide and a conventional PCW obtained by removing one row of air holes along the TK direction in a triangular-lattice PC. Effective mode transformation is achieved with a spot-size conversion ratio of 12:1. The low transmission efficiency (30%) that occurs in a direct coupling from dielectric waveguide into the PCW is raised to ~80% (0.98 dB) over a broad frequency range. Further improvements for nearly perfect transmission appear to be feasible.