Abstract: Approximately 60～70％ of the amphibole gabbro in Pingtan Island of Fujian Province is composed of bytownite (Pingtan bytownite). The wavelength dispersive analysis results by means of electron microprobe indicate that the composition of this bytownite ranges from An84 to An86. The SAED shows that the lattice of the Pingtan bytownite is primitive because the a, b, c, d diffractions all appear, but the c and d diffractions are diffuse and elongated along f axis. The relative intensity of c and d diffractions changes with different selected areas. The electron microsoopic research for this crystal shows that there is a sort of modulation structure in very fine scale all over the crysta1. Its elongated direction is nearly parallel to (010). Also the microtwin after albite law is commonly seen. In addition the low angle grain boundaries, which developed from the dislocation arrays, exist widely. These grain boundaries are the channels of the magmatic hydrothermal solution. The phyllosilicate grew within these channels. The cleavages of the phyllosilieates are all parallel to the channels. The genesis of the modulation structure is explained in this paper. It is believed that this is a sort of compositional modulation. As a Ca-rich phase of Huttenlocher intergrowth, the An85～88 bytownite has be en essentially decomposed.