This paper studies the optimal sizing and the experimental verification of a vehicle-used hybrid energy storage system (HESS). To determine the optimized combination that maximizes the traveling distance at an FTP driving pattern, an exhaustive search method was employed. Under constraints of cost, vehicle acceleration, and HESS gross weight, the optimal sizing can be derived. Next, for the purpose of investigating the transient behavior of HESS, equivalent circuits of the lithium battery set and ultracapacitors are proposed. Parameters of all components in circuits can be identified via Nyquist empirical formula and experimental data. An HESS testing platform was established for performance verification. It consists of an HESS, a battery control unit (BMS), power circuits, and an DC Electronic Load. With a supervisory computer, control variables can be adjusted online, while testing data can be retrieved. The results show that using HESS, system performance and battery cycle life will be improved.