Potassium channels are transmembrane proteins that enable the flux of potassium ions across the lipid membranes. They are characterized by high conductance rates near the diffusion limit with K+ selectivity more than a thousandfold over Na+. The potassium-selective NaK channel mutant “NaK2K” was found permeable for Rb+ and Cs+ at poor rates despite the sub-angstrom difference in the atomic radius between those ions and K+. Although experimental studies have characterized the conductance and binding profiles of different monovalent ions to NaK2K, the reasons behind this remarkable K+ selectivity is not clear yet at the atomistic level. Aiming to delineate the mechanistic of K+ selectively in NaK2K, we performed a microsecond scale computational simulations of Rb+ and K+ permeations through NaK2K using two different forcefields. Consistent with the previous experimental studies, our simulations showed very poor permiation rates of Rb+ compared to K+ regardless of the force field used. Both of the ions were found to have different binding profiles to the NaK2K selectivity filter. To further investigate the reasons behind the low permeation rates of Rb+, another two sets of simulations were carried out after modifying the LJ potentials parameters of Rb+ to those of K+. Modifying both of the LJ parameters independently did not impact the Rb+ permeation rates. On the other hand, modifying both of them at the same time resulted in permeation rates of Rb+ similar to those of K+. These observations suggest that the ion mass has no effect on its permeation rate but that it is dominated by precise coordination geometry.