Energy storage source is one of the key factors constraining the development of hybrid drive technology. Single energy storage source is difficult to satisfy the hybrid vehicle's requirements for both energy density and power density. This paper presents a hydraulic/electric synergy system (HESS) for heavy hybrid vehicles to overcome the existing drawbacks of single energy storage source. The key components in the synergy system are sized to improve the fuel economy potential while satisfying the vehicle performance constraints. In order to achieve optimal fuel economy, energy control strategy tailored specially to the synergy system is designed to manage the power distribution between multiple energy sources based on theirs characteristics. The experiments and simulations demonstrate that the proposed synergy system can provide good fuel economy and overall system efficiency. (C) 2010 Elsevier Ltd. All rights reserved.