Platinum hot-film sensors, whose typical size is 500nm wide x 10μm long x 40nm thick, are developed for investigating micro and nanoscale thermal events. This paper reports four kinds of applications with measurement principle, sensitivity analysis, and test results. Thermal conductivity of individual nanowire, represented by carbon nanotube, has been measured by bridging the specimen between the sensor and a heat sink. Here a new device is newly developed, which enables us to measure quantitatively both intrinsic thermal conductivity of the specimen and thermal contact resistance between the specimen and a target material. Focused beam-induced deposition is also measured by comparing the thermal conductance of deposited sensor and pristine one. Flow sensor is another application and we investigate the performance of carbon nanotube (CNT) fins deposited on the sensor by using dielectrophoresis technique to enhance the flow signal. The applicability of this sensor for measuring the thermal conductivity of fluid of very limited volume (femto-liter order) is also analytically studied.