Abstract: Cobalt nanoparticles (NPs) catalysts are extensively used in heterogeneous catalytic reactions, and the addition of alkali metal promoters is a common method to modulate the catalytic performance because the catalyst’s surface structures and morphologies are sensitive to the addition of promoters. However, the underlying modulation trend remains unclear. Herein, the adsorption of alkali metal promoters (Na and K) on the surfaces of face-centered-cubic (FCC) and hexagonal-closest packed (HCP) polymorphous cobalt was systematically investigated using density functional theory. Furthermore, the effect of alkali promoters on surface energies and nanoparticle morphologies was revealed on the basis of Wulff theory. For FCC-Co, the exposed area of the (111) facet in the nanoparticle increases with the adsorption coverage of alkali metal oxide. Meanwhile, the (311), (110), and (100) facets would disappear under the higher adsorption coverage of alkali metals. For HCP-Co, the Wulff morphology is dominated by the (0001) and ( 10\bar11 ) facets and is independent of the alkali metal adsorption coverage. This work provides insights into morphology modulation by alkali metal promoters for the rational design and synthesis of cobalt-based nanomaterials with desired facets and morphologies.