A new method of S-parameter extraction from the FDTD in analyses of microstrip circuits is proposed. It only uses the Ex-field component parts in three selected transverse cross sections of the transmission line under approach. The reflection coefficient is extracted from a single run of the FDTD. Neither prior knowledge of the line nor any computation of the derivative of the electric and magnetic fields is needed. The proposed new method is directly applicable to analyses of the discontinuities of microwave transmission circuits.