Abstract: It is time-consuming to calculate the transmission parameters of the rectangle defected ground structure(RDGS)using the conventional finite difference time domain method(FDTD)due to the Courant-Friedrich-Levy(CFL)limitation.The Crank-Nicolson FDTD(CN-FDTD),which is an unconditionally stable scheme,is adopted in this paper for improving the efficiency of the computation.The efficiency and the precision are analyzed in detail with the different time steps.The numerical results show that the CN-FDTD scheme is stable and efficient,even if the time step is far beyond the CFL limit.The results with the CN-FDTD show good agreement with the ones using the conventional FDTD,and the efficiency of the CN-FDTD scheme improves by 77.2%.In addition,the results using CN-FDTD is much more accurate than the ones with ADI-FDTD if the same time step is utilized.