Unlike the potassium ion channels, the selectivity filter of a NaK channel which conducts both sodium and potassium ions, contains a vestibule in the region corresponding to the first two binding sites of a potassium channel, in which ions can diffuse but cannot bind specifically.As the NaK crystal structures have only been obtained in Ca2+ rich environment with a Ca2+ blockage at the external site of the selectivity filter, detailed binding states of these two ionic species and how ions line in the filter in a normal state remain completely elusive.Here we find by extensive molecular dynamics simulations that K+ ion tends to bind at the center of a cage of eight carbonyl oxygen atoms of the filter, whereas Na+ ion tends to bind at the center.