Antireflection coatings fabricated between the substrate and the diffractive microstructure are shown to reduce Fresnel losses effectively, especially for high-index substrates used in the infrared region, if the diffractive structure and the antireflection stack are designed simultaneously. A substantial reduction of the Fabry-Perot effect caused by the high-index substrate is observed by using antireflection layers with films thicker than the normal quarter-wave films.