A constant temperature type of flow sensor using a solid state micromachining technology was developed for measuring the velocity of gas or liquid. It was designed to detect only the heat convection related to flow velocity. Other heat transfer terms and common mode interferences are canceled by differentiating both reference and exposed flow sensor. It employs the principle that the change of current through the sensing element can be used to measure the flow velocity. An experimental study of the behavior on this flow sensor was performed in a narrow tube(diameter : 8mm) for city water. The relation between power consumption of the flow sensor and square-root of flow velocity is almost linear in the low velocity range(0-200 cm/sec).