This paper investigates the development of a deaeration device to remove nitrogen from the hydraulic fluid in hydraulic hybrid vehicles (HHVs). HHVs, which use accumulators to store and recycle energy, can significantly reduce vehicle emissions in urban delivery vehicles. In accumulators, nitrogen behind a piston cylinder or inside a bladder pressurizes an incompressible fluid. The permeation of the nitrogen through the rubber bladder into the hydraulic fluid limits the efficiency and reliability of the HHV system, since the pressure drop in the hydraulic fluid can in turn cause cavitation on pump components and excessive noise in the system. The nitrogen bubbles within the hydraulic fluid may be removed through the employment of commercial bubble eliminators if the bubbles are larger than a certain threshold. However, gas is also dissolved within the hydraulic fluid; therefore, novel design is necessary for effective deaeration in the fluid HHV circuit. The following study employs a deaeration apparatus in parallel to the low pressure accumulator. Through an orifice, the pressure of hydraulic fluid is dropped to atmospheric to allow nitrogen bubbles to form coalesce and be separated from the hydraulic fluid. A screen mesh was added to slow down the flow of the foamy liquid and facilitate separation of bubbles. The nitrogen concentration in the hydraulic fluid before and after deaeration was evaluated using a Van Slyke device. A systematic study revealed that the screen did not have much effect; rather, the key variable was the pressure difference across the orifice. At typical operating conditions (690 kPa, with 138 kPa saturation in the low pressure accumulator), the concentration of nitrogen is reduced by 68±1%, dropping the concentration level to less than that of fluid saturated at atmospheric pressure.