Provided are a method of adjusting a photonic bandgap of a photonic crystal, a method of manufacturing a reflective color filter using the same, and a display device including the reflective color filter. The method of adjusting a photonic bandgap of a photonic crystal includes forming the photonic crystal having a photonic bandgap on a substrate, and changing the photonic bandgap by irradiating light onto the photonic crystal. In addition, the display device includes a backlight, a transflective liquid crystal panel including liquid crystal cells sealed between first and second substrates. Each liquid crystal cell corresponding to a pixel includes a transmissive area and a reflective area. A transmissive color filter is formed on the first substrate, which faces the backlight, and a reflective color filter is formed on each reflective area.