We report here temperature measurement by means of a Ga-filled C nanotube thermometer with diameter,150 nm and length; 12mm. The method relies on the initial identification and calibration of a nanothermometer in a transmission electron microscope TEM, followed by placing it into an air-filled furnace whose temperature is to be measured, and final TEM reading of a postmeasurement gradation mark visible inside the tubular channel. The mark originates from the fact that, at high temperature, the Ga column tip exposed to the air through the open C nanotube end oxidizes, and a thin Ga oxide layer sticks to the nanotube walls upon cooling. The temperature according to this gradation mark coincides closely with nominal furnace temperature controlled by standard means. The method paves the way for practical temperature measurements using a C nanothermometer in air and within spatially localized regions e.g., dimensions of tens of micrometers.