The emission properties of the polymer light-emitting devices MEW fabricated by the maskless dye diffusion technique in correlation with the diffusion degree have been studied. It has been found that the emission intensity at a constant voltage slightly increased by the dye diffusion, however, at this stage, no improvement was observed in the external quantum efficiency. The emission color of the devices can be tuned to pure color from the dye diffused at a certain diffusion degree, but further diffusion induces critical damage to the device, indicating that a certain optimum diffusion degree exists. These observations suggest that the optimal condition for the maskless dye diffusion must be considered not only from the heating condition but also from the dye concentration in the reservoir film.