The development of a theoretical model for fluidelastic instability (FEI) in tube arrays is presented. Based on the simple model of Lever and Weaver, it considers a group of seven tubes which move in both the streamwise and transverse directions. The analysis does not constrain either tube frequency or relative mode shape so that the tubes' behavior evolves from a perturbation naturally. No additional empirical input is required. A particular case is used to evaluate the model's performance and the ratio of streamwise-to-transverse natural frequency is varied. Both streamwise and transverse directions of FEI are predicted and the results agree well with experimental observations.