Micro‐cavities in two‐dimensional silicon‐on‐insulator (SOI) photonic‐crystal slabs are studied. A combination of the finite‐difference time‐domain (FDTD) technique and the Padé approximation with Baker's algorithm is used to accurately compute the resonant frequencies and quality factors simultaneously. The simulations show that, for similar cavity structures, the mode quality factors of cavities in SOI‐type photonic‐crystal slabs are much smaller than those in membrane photonic‐crystal slabs, and deep etching of air holes is still required to obtain relatively higher Q. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 45: 381–385, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20830