Recent developments of smart materials such as piezo devices have been applied to small hydraulic pumps, enabling to meet various demands. For the compact pump, the valves are critical components in the aspect of fluid dynamics. In this study, the flow inside the reed valve port driven by the piezo actuator was experimentally observed. When a sine wave with a driving frequency of 90 Hz was applied, the maximum flow rate could be obtained. It was found that the developed flow opposite to the outlet direction at the root portion of the valve prevented further increase of the flow rate according to the operating frequency.