In the last two decades, we are facing air pollution problem and the green house effects which are successive results of increase in CO 2, Particulate Matter and so on. This leads to the development of the hybrid vehicle and the fuel saving has been realized. In this research, the hybrid vehicle using Constant Pressure System (CPS) is discussed. CPS keeps the line pressure constant by regulating the output flow rate from energy storage device according to the requirement of load. The vehicle is accelerated with energy fed from the high-pressure line to the drive pump/motor while the kinetic energy of the vehicle is stored in flywheel by working the drive pump/motor as a pump during the deceleration. In this paper, first the driving principle of the vehicle used in this study is explained, and the flywheel system is discussed. Then the vehicle speed control performance is examined on real machine and the method for improving the fuel economy is also discussed.