The iridescence and microstructure of nacre in Perna viridis are investigated by fiber spectrometer, scanning electron microscope (SEM) and theoretical simulated software. The following results are obtained: nacre has a typical one-dimensional photonic crystal structure that consists of aragonite sheet and organic matrix; thickness of aragonite sheets gradually increases from 392 nm in growing region to 537 nm in mature region of nacre, which leads to an obvious red shift of photonic band gaps with the same order; dominant wavelength of different regions of nacre increases from 480 nm (blue) to 527 nm (green). The results of measured reflection spectra and theoretical simulation indicate that the 2~4 order photonic band gaps of nacre are in ultraviolet, blue and red band, which cause the complicated iridescence of nacre.