The dynamic responses of a Li-ion battery pack deployed on hybrid electric vehicles are studied with a high fidelity finite element model and a parametric reduced-order model. The effects of microstructure transformation in the electrode materials caused by lithium-ion intercaladon/deintercalation on the evolution of dynamic responses are investigated including the effects of the state of charge, aging, and cell-to-cell variations. The dynamic responses obtained from a finite element analysis show two interesting phenomena. First, the high modal density is controllable with the design modification of a pack component. Second, dynamic responses, especially the evolution of the natural frequencies of the fixed-boundary modes, of a Li-ion battery pack provide useful information to estimate the dynamic states or health states of the battery. A probabilistic analysis is also carried out considering stochastic operational conditions of hybrid electric vehicles with a parametric reduced order model. The probabilistic analysis not only suggests appropriate modes and locations for monitoring its dynamic responses, but also determines the maximum response level of every cell in the battery pack. The proposed modeling approach can improve the safety and reliability of the structural design of battery cells and packs. Furthermore, it can be useful for the identification of the battery states during the operation.