In the analysis for durability or R&H performance with the full vehicle multibody models, the need for component flexibility is increasing along with demand for more precise full vehicle system. The component elastic deformations are usually expressed by modal superposition from component normal mode analysis with finite element model for reducing model size and simulation time. Although the simulation results of MBD analysis are more accurate according to increasing the number of flexible body and modes, the increasing of flexible components makes worse simulation time and convergence in MBD analysis. Especially, in the MBD analysis including a flexible upper body, in substitution for large number degree of freedom FE model such as trimmed body, it should take a few times longer than the case of rigid upper bodyThis paper proposes the methods of reducing computational cost with adequate mode selections without the loss of simulation accuracy in the flexible MBD. An optimal modes selection method employs modal participation factor calculated with only modal deformation shapes at the interface nodes of flexible body. Examples are provided that demonstrate the performance of the method and also how to select essential modes for using only external (interface) modes shape information.