The dynamic behavior of a very large rectangular floating structure is considered. The structure is modelled as a plate with free edges. Two different thicknesses are considered for the model. The Mindlin plate theory is used to formulate the structure behavior. Natural frequencies, mode shapes, and stress resultants of the structure are predicted by using finite element method. For this purpose, a MATLAB code is written. The same analysis is performed by using the ANSYS software. The results of these two analysis are compared with each other and with the available results in the literature, where close agreement is observed. Therefore, the written finite element code is found to be acceptable for prediction of the dynamic behavior of very large rectangular floating structures in early stages of design.