Two isoparametric finite element models (QS8S2 and QL9S2) have been proposed for the static and vibration analysis of plates / shells with arbitrarily positioned eccentric stiffeners. The eight-node QS8S2 model and the nine-node QL9S2 model have been achieved by properly combining Serendipity and Lagrangian plate / wrap elements with the three-node isoparametric beam element, using appropriate transformations for eccentricity and arbitrary location in the plate element. This has been achieved without increasing the total number of global degrees of freedom (DOF). Transverse shear deformations are included in the formulation, making the models applicable to both moderately thick and thin plates. Numerical studies have been performed for the static and vibration analysis of concentric and eccentrically stiffened plates to determine the efficiency of the proposed models. The vibration analysis has been carried out with four mass-grouping systems. It has been found that the proposed QL9S2 model with a consistent diagonal mass grouping scheme is more efficient than the QS8S2 model and the other models available in the literature for static and vibration analysis (in the higher frequencies / modes range) of arbitrarily positioned stiffened plates / stiffeners.