Nonreciprocity is the main contribution of drift error in fiber optic sensors based on Sagnac interferometer, such as the fiber optic gyroscope. When the sensor has to suffer from wide range of temperature, thermal nonreciprocity becomes the most important one required to overcome for it to retain similar performance over all application condition. In this paper, thermal nonreciprocities induced by both pure temperature and thermal stress in the Sagnac interferometer are analyzed respectively. First we deduce the expressions of nonreciprocities in form of phase error of the Sagnac interferometer’s fiber coil, induced by pure temperature and thermal stress respectively. Then a FEA model of such a fiber coil is build up, based on which pure temperature nonreciprocity and thermal stress nonreciprocity are investigated in detail, and their numerical values are also calculated. Finally an experimental system is set up to verify the numerical results and the corresponding errors of the system is measured, which shows good agreement to the simulation value. At the end of this paper, we discuss some methods for reducing the thermal nonreciprocity in the fiber coil, some of which are proposed for the first time.