The adsorption mechanisms of elemental arsenic and selenium on the CaO (001) surface were studied with periodic slab models based on density functional theory. The results showed that As and Se were strongly adsorbed on O at the top of the CaO surface with maximum binding energies of -2.07 and -2.92 eV, respectively. When the surface coverage was increased from 0.056 to 0.125 monolayer, the binding energies decreased slightly. Analysis of the electronic partial density of states showed that adatom states strongly hybridized with O 2p states to form As-O and Se-O covalent bonds when As and Se were adsorbed on the O top site. In contrast, As-Ca and Se-Ca ionic bonds were formed on the Ca top site as well as the O top site. Population analysis showed that the adsorption of As and Se on the O top site reduced the strength of the substrate Ca-O bonds.