In hybrid electric vehicle (HEV) design and operation, no parameter plays a more important role than efficiency. Unlike conventional vehicles, HEVs are generally energy and power limited by the battery bank and auxiliary power unit. As a result, the overall system efficiency in the conversion of chemical or stored energy into kinetic energy of the vehicle is the key parameter that drives the overall system design.We have undertaken a sensitivity analysis of HEVs to understand in detail the various factors and their respective weights that affect the overall system efficiency. Our goal is to identify the parameters that most significantly influence vehicle efficiency. The results of this study may be used as a guide to focus work on the areas of most benefit for HEVs as well as aiding in sizing vehicle components for maximum efficiency. The simulation tool used is SIMPLEV 3.1 developed by the DOE Idaho National Engineering Laboratory, which runs a time-based simulation on a set of parameter files for an (hybrid) electric vehicle. To accomplish this sensitivity analysis we have written a set of programs that conduct a parametric analysis of the vehicle parameters by creating the necessary sets of SlMPLEV input files. It then runs these SIMPLEV files on a standard test cycle and extracts out the results of the simulation.Using the data this generated, we were able to establish the relative importance of each system parameter. This paper discusses the assumptions within the SIMPLEV model, the development of input files and the parametric programs, and the test cycle chosen. The results of this sensitivity analysis are also presented and analyzed.