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A novel small monolithic gas flow sensor has been designed and fabricated by use of micromachining of silicon. Its operation is based on the cooling of an electrically heated mass by the gas flow, and detection of the mass's temperature by a diode. The small size, 0.4 mm by 0.3 mm by 30 µm, of the hot part of the sensor gives a fast thermal response (time constant 50 ms). By using polyimide as a thermal insulator a high gas flow sensitivity is achieved, The shape of the sensor will present very little obstruction to the gas flow and also makes it easy to mount.