The structure of the high magnetic permeability Pe-Si-Al alloy containing about 10 percent Al and 5 percent Si has been investigated. By using the foil specimen technique of electron microscope, the structure of matrix was found as ordered fcc Fe3(Al, Si) phase, its lattice parameter being 5.70±0.03?. This value lies between the values of the lattice parameter of Fe3Si and Fe3Al. This explains that a proportion of the silicon atoms is replaced by the aluminium atoms. Three sets of electron diffraction patterns of the matrix combined with the precipitated phase were obtained and their orientation relationships determined.The carbon replica of the extracted precipitated phase in selected regions is diffracted by contrast in the electron microscope. The Fe3(Al, Sl) Cx phase is a cubie structure. It might be suggested that the Fe atoms occupy the positions of face centres, Al(Si) atoms occupy the cube corners, and the carbon atoms occupy the body centres.