Abstract: In this paper using the method of equivalent currents (MEC) and physical theory of diffraction (PTD) electromagnetic back-diffraction field, which depends on geometry of the edge and direction, polarization and wavelength of the incident wave, is studied. Some features of back-dif f rat ion field of the edge are summarized in the conclusion. The conclusions are very useful for studying radar cross section (RCS) of complex targets, especially for the low observable aircraft.