The effect of excess Nb2O5 on the phase stability of tungsten-bronze-structured K3Li2Nb5O15 (KLN) ceramics was studied. Stoichiometric KLN ceramics are not obtained as a single phase and second phases of KNbO3 and Li3NbO4 were observed. Additionally, stoichiometric KLN is difficult to sinter. In Nb-rich compositions, the second phase disappeared and a single KLN phase was obtained. This phase development behavior, that is, the phase stability of the KLN, was analyzed from the viewpoint of the electrostatic potentials of ions. The calculated Madelung energy of the completely filled stoichiometric KLN was unstable, while Nb-rich compositions showed much reduced Madelung energy, indicating that the ions were stabilized electrostatically. Enhanced sinterability in Nb-rich compositions is also discussed.