A semiconductor flow velocity sensor having a semiconductor substrate and an electrical heating unit arranged on the substrate. The sensor located in the fluid flow to be measured can electrically detect the radiation from the heating unit which varies depending on the velocity of fluid flow. Thermal insulator is provided between the semiconductor (silicon) substrate and the heating unit so as to thermally shield the substrate from the heating unit. On the thermally isolated area of the substrate is provided an external sensor circuit so as to enable to increase the integration of the sensor.