Abstract: We carried out first-principles calculations of Li (110), (100) and (111) free-standing thin films from 3 to 30 monolayers to study oscillatory quantum size effects exhibited in surface energy, adsorption energy of hydrogen atoms. Furthermore, we investigated adsorption height of hydrogen atom, density of states at Fermi level and work function of Li (110) thin films. Calculated physical quantities feature clearly quantum oscillations as functions of film thickness, which is also called quantum size effects. Calculated results show clearly quantum oscillations in adsorption energetics, which can be used to modulate chemical reactivity and other surface process in nanostructure materials. Finally, calculated results show clearly that work function of Li (110) films is reduced about 0.9 eV by adsorption of hydrogen atoms. Adsorbed hydrogen atoms cut down electrostatic potential of Li atoms on the outermost layer and vacuum layer, resulting in decrease of work function with adsorption of hydrogen atoms.