Abstract: According to the physical definition of diffracted wave, the solution of Kirchhoff-Helm-holtz diffraction equation at a fault give by Trorey is simplified and developed. Synthetic seismograms of diffracted wave at a fault of different depths and from different directions to the profile are computed, and compared with corresponding reflected wave. The amplitude spectrum and phase spectrum of diffracted wave on both sides of a fault are computed. Some usable formulas using diffracted wave along a profile for locating a fault are presented. Two practical examples are given for studying particular characteristics of diffracted wave at a fault.Results indicate that the diffracted wave at a fault originates from an abrupt change in the medium. On both sides of a diffraction point, the travl-time curve of diffracted wave is a hyperbola. The first arrival of the diffracted wave is distinct and with short half-period, and its amplitude spectrum at predominant frequency is similar to that of reflected wave. It is noted that at the tangent point of the travel-time curves of diffracted and reflected waves, the amplitude of the diffrated wave is maximum, but the phase is opposite to that of the reflected wave. The apparent diffracting point does not coincide with the real one. These charateristics might offer a criterion for identifying the diffracted waves and the position of a fault.