AbstractThe weakly conditionally stable finite‐difference time‐domain (WCS‐FDTD) method is employed to compute the electromagnetic coupling and shielding of various small aperture‐coupled metallic enclosures. In the WCS‐FDTD method, a larger time‐step size than allowed by the CFL stability condition limitation can be set because the algorithm of this method is weakly conditionally stable. Consequently, an increase in computational efforts caused by fine cells due to small apertures can be prevented. The results from the WCS‐FDTD method agree well with results from the conventional finite‐difference time‐domain (FDTD) method, and the required CPU time for the WCS‐FDTD method is much shorter than that for the FDTD method. Compared with the ADI‐FDTD method, the WCS‐FDTD method has better accuracy and higher efficiency, which is demonstrated by numerical examples. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1852–1858, 2007; Published online in Wiley Inter‐Science (www.interscience.wiley.com). DOI 10.1002/mop.22633