The co-adsorption of titanium atoms and oxygen molecule on the Ag(100) cluster surface has been investigated by the density functional theory calculation. In order to achieve the convergence of a self-consistent-field (SCF) iteration, the approach using the pseudo-fractional occupation number (pFON) or the quadratically convergent SCF (QCSCF) approach is employed in this study. First, we investigated the adsorption of nTi atoms (n = 1-4) on Ag(100), and found that any bonding orbitals between Ti and Ag atoms were not formed and the systems were stabilized by the formation of Ti—Ti bonds. As the Ti-precovered surfaces were interacted with O₂, we found that oxygen adsorbs both molecularly and dissociatively at n ≤ 2, while the adsorbed oxygen always dissociates at n ≥ 3. At n ≥ 3, the dissociated O atoms form a TiO-chain or a TiO-cyclic unit. We also revealed that the formation of Ti—O—Ti bond is essential for the stabilization of O₂/nTi/Ag(100) systems. [DOI: 10.1380/ejssnt.2009.7]