This study investigates a simple method for thickness estimation with single layer accuracy for self-assembling opal crystals prepared by the dip-coating method. The thickness (number of layers) of the opal crystals was estimated by an analysis of the optical reflectance from s-polarization incident light, and then verified with SEM. The opal crystals were considered as periodic dielectric layers and were analyzed with the transfer matrix method. The reflectance simulation showed a good agreement with the experimental results. The lattice constant and the thickness were determined at the peak position and by the fringes of the reflection spectra, respectively. (C) 2011 Elsevier Inc. All rights reserved.