With the popularity of high pressure hydraulic system and the increasing requirements for the efficient and automatic control, the measure of tiny flow under the high pressure condition is more and more important. And the relevant research also has great importance for the development of domestic flow meters industry. A bi-directional oval gear flowmeter used for the measurement of tiny flow under high pressure condition was presented, which used a pair of low eccentricity oval gears as the core components and used hall sensors for the non-contact signal-detection. The designed flowmeter was with a lower displacement in order to measure tiny flow. A fully closed measurement volume was built so that the flowmeter can endure high pressure. A complete CAD model was established for the CNC machining of oval gear in order to ensure the high accuracy, including the pitch curve equation of oval gear and the solution of tooth profile based on the converted method. In order to validate the design and research the flowmeters metrological characteristics, experiments on a prototype fabricated were performed. And a modification method of flow coefficient was proposed on the basis of least square method. The experiments results showed that the maximal pressure loss was 0.46MPa at flow rate of 3L/min, and the flowmeter was with a relative error less than 1% over a flow rate range of 0.03~3L/min, and 0.5% over a flow rate range of 0.05~3L/min.