Abstract: In order to investigate the dynamical contribution of individual component and equipment in system-level models, this study proposed a computational method based on the modal strain energy (MSE). Two engineering case studies applying the MSE method are demonstrated herein. The first study investigates the main reason causing large seismic response change of a prototype reactor coolant system (RCS) surge pipe due to the location change of surge suspension device. The analysis shows that the local dominant mode of the surge pipe is altered by the location change of the suspension. Such change of dominant resonant frequency leads to a significant change at the corresponding frequency of input seismic response spectrum, thereafter affects the dynamical response. The second case study discusses the decoupled strategy of the main steam-pipe from the loop model for separated dynamical analyses. MSE analysis, together with the second decoupling criteria suggested by USNRC SRP 3.7.2, justifies this deed. Modal strain energy (MSE) method proposed in this paper is able to quantify the dynamical contribution of components and equipment from the system-level dynamical analysis models. The computation of MSE only utilizes the system-level model stiffness and mass matrices. No time-domain transient analysis is necessary.