Abstract An optical method for high-temperature thermometry in heavy radiation fields has been demonstrated. The measurement system consisted of signal generator, transmitting optical fiber and optical detector. The signal generator was a small blackbody cavity and the radiance from the cavity was used to determine the temperature. The method is applicable to high temperature measurements of small material samples in reactor environments also in strong electromagnetic fields. The theory of operation was confirmed in an experiment at Japan Materials Testing Reactor where the tip of the transmitting pure-silica-core fiber itself served as blackbody cavity and the temperature of a small alumina sample was determined by optical means. It was shown that thermally powered infrared emissive sensors have the ability to be used for the full lifetime of the transmitting optical fibers as temperature data could be obtained for 2 full-power (50 MW) weeks. In the performed experiments the heating was induced mainly by γ -radiation and optically measured temperatures ranged from 250 to 800°C. The demonstrated method relies on simple and compact sensors that require no electro-optical sources and can be used in environments where both nuclear radiation and electromagnetic fields are prevalent for extended periods of time.