A hybrid electric vehicle (HEV) system of heavy duty truck (HDT) for long haul application was studied to improve the fuel economy (FE). At first, the HEV system configuration and control functions were defined and optimized. Secondly the test vehicle was constructed and a test drive was carried out on Tomei express way, and then, 10% FE improvement compared to non-hybrid HDT was achieved.Additionally, HEV system with coasting technology, which had become attractive as a FE improvement technology for non-hybrid HDT, was considered to obtain further FE benefit. The effective usage of both “Electrical energy regeneration control” for HEV system and “Kinetic energy control” for coasting was studied with simulation. As a result, it was clarified that the effects for FE improvement depended on route profile and those combined operations advantaged for FE improvement compared to each individual operation.