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Wave plates with subwavelength structures are fabricated by applying semiconductor processes to realize a high-aspect-ratio grating structure with a small linewidth and a large grating step. We have fabricated wave plates with a large phase retardation using a titanium dioxide (TiO 2 ) film. However, the plates have a problem in that a slight deviation in grating step will bring about a large difference in phase retardation because TiO 2 has a large refractive index. To solve this problem, heat treatment of the fabricated wave plates was carried out to change the refractive index. As a result, the relationship between the heating temperature and the increasing ratio of phase retardation was determined, which led to the successful control of the optical properties. The effective temperature was 672 to 1072 K through the characterization of the heat-treated plates. By utilizing this process, the yield of manufactured wave plates can be increased greatly.