A Maoh-Zehnder interferometer using single-mode fibers has been built, and its temperature and heat radiation sensitivity has been investigated. The results are that the temperature responsivity and the heat radiations responsivity are about 110 rad/m℃ and 3.7 × 10-5 rad/m erg respectively. In heat radiation measurement, the absorbed heat radiation energy in the optical fiber is estimated to be 5-8% of the total energy passing the receiving area.