A theoretical model of the fluidelastic instability in tube arrays is presented in this article. It is developed for a normal-square cylinder array and then extended to other types of array patterns. The model is based on transient interactions between a single cylinder and the adjacent flow streams of single phase fluid. The central cylinder is assumed to oscillate as a one-degree-of-freedom mass on a spring system in the lift direction only. A small displacement of cylinder is assumed to perturb the surrounding interstitial flow, while as for higher displacements the cylinder causes flow distortions in regular intervals. These disturbances are convected downstream along with the interstitial flow. The waveforms of these flow distortions are assumed to interact with the array pattern, thence modifying the fluid force acting on the cylinder. The critical flow velocity is obtained as a function of mass ratio and damping parameter. The proportionality constant of the mathematical model is derived in terms of the pitch ratio and Euler number. The mathematical development results in an implicit model for the critical flow velocity. The model predictions are in a good agreement with experimental results.