The iridescent blue color of Morpho butterfly can be found from far. This is a typical structural color, mainly caused by several elementary optical processes with the ordered arrays of micro/nano structures including multilayer interference, diffraction grating, light scattering, etc. A general design tool based on the rigorous coupled-wave analysis (RCWA) for diffractive optical structures is employed to construct a two-dimensional model that mimicks the wing scales. The influence of the structural parameters on optical properties is systematically analyzed, including the horizontal duty ratio of the bottom dielectric element, the width difference of the adjacent dielectric elements, the vertical duty ratio of the dielectric element, and the angle between the spine and the base. The mechanism of the structural color is then revealed. The results can be used to steer the design, manufacture and applications of the bionic micro/nano structures/devices for environmental detection.