We have examined the diffusion and agglomeration of Au adatoms on the H-terminated Si (111) - (1×1) surface using periodic slab density functional theory calculations. We find that a single Au atom favorably resides atop a surface Si atom by breaking an original ≡Si-H bond while the H atom is bonded to the Au atom in the vertical direction, leading to the ≡Si-Au-H state. Starting from the most favorable on-top (T) site, a Au adatom is predicted to undergo diffusion by moving in and out of the T site without disrupting surface Si-H bonds. The predicted overall activation energy for the Au diffusion is 0.5 eV. Our calculations show that Au agglomeration leads to libration of H atoms from the Au/Si interface, while the H atoms are weakly bound to Au clusters and subsequently undergo associative H 2 desorption with no significant barrier. Based on charge density analysis we also discuss bonding mechanisms for Au on H-terminated Si (111) - (1×1). Our findings are as a whole consistent with experimental results available in literature.