The　adsorptions　of　a　series　of　alkali　metal　(AM)　atoms,　Li,　Na,　K,　Rb　and　Cs,　on　a　Si(001)-2　X　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.