A heuristic constitutive equation and an FE formulation for rubber under small oscillatory loads superimposed on large static deformations are presented. The viscoelastic constitutive model, , is implemented in an FE code to analyze the dynamic characteristics of rubber elements under general loading conditions. Dynamic tests in which the rubber specimens endure steady-state harmonic motion superimposed on large static deformation are performed in order to verify the proposed model. Compression and complex-stress tests are included in the tests to check the proposed model under multi-axial stress states. The FEA results are compared with the experimental results. The proposed model successfully predicts dynamic stiffness peak in the complex-stress test, which cannot be explained by conventional models. The model shows a better performance than existing models in predicting the behavior of rubber specimens that are subject to complex pre-strain.