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The specific absorption rate (SAR) is an indicator of the thermal effects caused by microwave exposure in biological tissues. Electric field probe scanning has been used as a practical SAR evaluation technology until now. A measurement standard is necessary for any physical quantity. In general, the existence of a standard constructed with a physically different procedure is important in uncertainty evaluation of the measurement standard. In this paper, a thermal SAR measurement method using an optical fiber thermal sensor is proposed. The thermal SAR measurement method is not suitable for dosimetric assessments for an actual cellular phone. Nevertheless, the proposed method is a simple measurement technique for measuring the temperature rise caused in an equivalent biological tissue (semisolid phantom) by electromagnetic energy absorption. A thermal sensor works independently of the frequency and is, therefore, suitable for primary testing of an electric field probe scanning system. The measurement utility of the proposed thermal SAR measurement method was verified in this paper. The results show that the proposed method is able to measure SAR within an error range of 1%-3% above 2 GHz.