A modal finite-difference time-domain (FDTD) method is extended for the analysis of ridged cavities, which are uniform in the z-direction. Assuming that the end surfaces of cavity are the perfect conductor, thus, the fields along the z-axis can be described by kz. Therefore, three-dimensional (3-D) problems can be simulated by the use of a two-dimensional model. Besides, to achieve a faster computation, the field components are expressed by two pairs of equations—sine and cosine. To validate the utility and efficiency of proposed method, we analyzed two ridged cavities. Numerical results show that less than one-tenth memory and CPU requirements are needed by the modal FDTD as compared with conventional 3-D FDTD method. Copyright © 2012 John Wiley & Sons, Ltd.