Macroscopic and microcosmic observations of the plagioclases from plagioclasite of southern Finland indicate that the plagioclases with parallel needle-like inclusions has iridescence, while the plagioclases without inclusions do not have iridescence. The inclusions can be divided into two types based on their degrees of development. Dense-type inclusions are 35～50 stripes/mm with 0.01～0.03 mm in length and 0.004 mm in width, while sparse-type inclusions are 25～35 stripes/mm with 0.005～0.015 mm in length and also 0.004 mm in width. The plagioclases with dense-type inclusions are mainly blue while the sparse-type inclusions are orange and red in color. A compositional analysis by means of scanning electron microscope, electron microprobe and laser Raman spectrometry shows that the inclusions are magnetite in composition, and the plagioclases with or without inclusions have the same composition. It is suggested that the composition of the plagioclase and inclusions and the twin of the plagioclases are not the causes for the iridescence of the plagioclases, and the parallel distribution of the needle-like opaque inclusions in the plagioclase seems to be the factor responsible for the iridescent plagioclase. The needle-like parallel inclusions in plagioclases resemble the grating in physics. The iridescence of the plagioclase is probably generated by the diffraction and interference of the light.