Abstract: In recent years considerable research has been focused on distinguishing natural from synthetic opals. However, with regard to synthetic opals, less attention has been paid to investigate their microstructure, the formation mechanism of play of color and the form of water content. Fourier Transform Infrared Spectroscopy (FTIR) combined with Field Emission-Scanning Electron Microscopy (FE-SEM) were used to systematically investigate the microstructure of a synthetic opal. With the help of heat treatment, the formation mechanism of body color, play of color and the occurrence state of water have been preliminarily studied. Results show that the synthetic opal had typical absorption peaks at 2900 cm^-1 and 1737 cm^-1 in FTIR Spectroscopy. The absorption peaks of synthetic opal were more complex than those of natural opal-between 6000 cm^-1 and 4000 cm^-1 (in Fig.2). The color of synthetic opal consists of body color and play of color. The body color has a direct association with the substances filling in the pores among these quasi-spherical SiO₂ particles with a diameter of 205 nm, and it gradually disappeared as the substances escaped from the pores (in Fig.5). The play-of-color of synthetic opal originated from three dimensional photonic band gap structures, which consisted of periodicity-arranged SiO₂ particles. In addition, before the micro-architecture made by SiO₂ particles was broken, the main color depended on the particle size and the substances filling the pores.