The adsorptions of a series of alkali metal(AM) atoms, Li, Na, K, Rb and Cs, on a Si(001)-2×2 surface at 0.25 monolayer coverage have been investigated systematically by means of density functional theory calculations. The effects of the size of AM atoms on the Si(001) surface are focused in the present work by examining the most stable adsorption site, diffusion path, band structure, charge transfer, and the change of work function for different adsorbates. Our results suggest that, when the interactions among AM atoms are neglectable, these AM atoms can be divided into three classes. For Li and Na atoms, they show unique site preferences, and correspond to the strongest and weakest AM–Si interactions, respectively. In particular, the band structure calculation indicates that the nature of Li–Si interaction differs significantly from others. For the adsorptions of other AM atoms with larger size(namely, K, Rb and Cs), the similarities in the atomic and electronic structures are observed, implying that the atom size has little influence on the adsorption behavior for these large AM atoms on the Si(001) surface. The adsorptions of a series of alkali metal (AM) atoms, Li, Na, K, Rb and Cs, on a Si (001) -2 × 2 surface at 0.25 monolayer coverage have been investigated systematically by means of density functional theory calculations. The effects of the size of AM atoms on the Si (001) surface are focused in the present work by examining the most stable adsorption site, diffusion path, band structure, charge transfer, and the change of work function for different adsorbates. suggest that, when the interactions among AM atoms are neglectable, these AM atoms can be divided into three classes. For Li and Na atoms, they show unique site preferences, and correspond to the strongest and weakest AM-Si interactions, respectively. , the band structure calculation indicates that the nature of Li-Si interaction more significantly from others. For the adsorptions of other AM atoms with larger size (namely, K, Rb and Cs), the similarities in the atomic and electronic structures are o bserved, implying that the atom size has little influence on the adsorption behavior for these large AM atoms on the Si (001) surface.