Abstract: A yarn heater has an elongate, hollow metal yarn heating member with a substantially constant internal cross-section, an electric yarn heating element mounted externally of the heating member and a baffle located internally inside of the heating member extending longitudinally therein and dividing the inside space into two passageways. The heating member is filled with a heating fluid which when heated by the heating element consists of a liquid phase and a vapor phase. Located at the lower end of the baffle is an aperture for permitting automatic circulation of the heating fluid. A yarn heating surface located at the front of the heating member and extending along the length thereof is formed with a pair of longitudinal grooves for guiding the yarn.