Glycine adsorption on Si(1 1 1)7x7 and Si(1 0 0)2x1 model surfaces are studied by using Density Functional Theory (DFT) calculations. The present work illustrates that the formation of different surface adducts involving the -NH2 and -COOH functional groups plays a vital role in determining the adsorbate structures on the 7x7 and 2x1 surfaces. No Si <- O surface adduct can be obtained on Si(1 1 1)7x7, in marked contrast to Si(1 0 0)2x1, on which the formation of Si-O bond readily leads to O-H dissociative product. For Si(1 1 1)7x7, the existence of only Si <- N adduct leads to N-H dissociative product and the formation of Si-N bond. These results are in good agreement with the experiment. (C) 2011 Elsevier B. V. All rights reserved.