A numerical method for determining diffraction coefficients for edge of half-infinite metallic plate is described. We use the Finite-difference Time-domain （FDTD） method to compute the scattered field of an object possessing several scattering centers. The numerical diffraction coefficient of the scattering center under investigation is then isolated by solving simultaneous equations established based on GTD consideration. This method yields numerical diffraction coefficients for the edge of conducting strip and the one partially or fully coated by absorbing material for which the analytical diffraction coefficients are not known to our knowledge. Some examples show the feasibility.