We conducted structural characterizations, reflection measurements, and theoretical simulations on the iridescent green of the wings of Papilio bianor ganesa. We found that the green iridescence originated from a stack of about 9 periodic structure composed of chitin layers and air-spacer layers. This periodic structure gave rise to a high-reflection band in yellowish-green part of the visible spectrum under normal incidence. With increasing incident angle, all reflection peaks showed a blue shift in wavelength. Based on the structural characterizations, the theoretical simulations accurately predicted the shifts in spectral reflectivity which might be observed experimentally.