We have used in situ scanning tunneling microscopy (STM) to study the facet formation in the selective growth of pyramidal Si nanocrystals on Si(001) windows in ultrathin 0.3-nm-thick SiO2 films. Broad (001) surfaces developed as the top of the crystals, and {1, 1,(2n + 1)} (n = 1-6) facets formed the sidewalls. As growth continued, the slope angle of sidewall facets increased, and {1, 1, 9} and {1, 1,(2m + 1)} (0 < m < 4) facets often came to coexist on the sidewalls. On well-oriented Si(001) surfaces, layer-by-layer growth in the [001] direction was dominant. On vicinal Si(001) surfaces, lateral step growth took place in the initial stage, and the layer-by-layer growth was suppressed until after a large (001) surface had formed as the top of the crystal. (C) Elsevier Science B.V. All rights reserved.