The authors report the development of a dipole-coupling model to reveal the localized and propagating plasmon waves of two-dimensional ordered nanoparticle arrays. Such model accurately predicts the distinct optical behaviours of the arrays arranged in square and hexagonal lattices, which would not be possible with traditional effective-medium models. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)In this Letter the authors use a dipole-coupling model to derive the effective anisotropic dielectric response of ordered silver nanoparticle arrays. The resonant wavelength of the transverse localized plasmon mode (TP) of the square-lattice array starts deviating from that of the hexagonal-lattice array as the interparticle spacing (S) is less than 40 nm. This behavior evidences that this model more accurately predicts plasmonic interaction between nanoparticles than the effective-medium model.