In order to improve the fuel economy of the heavy duty trucks at a highway driving condition, the heavy duty hybrid trucks with new type of hybrid electric assist engine system were proposed at the previous report. The new system consists of a downsizing diesel engine with a two-stage charging structure, which has an electric supercharger with bypass circuit and a conventional turbocharger, the hybrid electric motor and the small-capacity battery. The electric power consumption of an electric supercharger is equivalent to the amount of the regeneration power produced during high-speed driving where the opportunity of the regeneration is small.In this report, an electric supercharger for the heavy duty hybrid truck was produced experimentally. First, the engine performance and exhaust emissions were investigated using the 4 cylinder diesel engine equipped with an electric supercharger. Next, the fuel economy of this heavy duty hybrid truck driving in a Japanese in-city mode and a Japanese interurban mode was investigated by using the “Hybrid Powertrain Test Bed System”.