The polarization-temperature characteristic of single-mode (SM) fiber coupler was investigated theoretically and experimentally.An analytic expression shows that the factors affecting the extinction ratio (ER) of the output light of SM fiber coupler include the extinction ratio of the input light and the mode-coupling ratio of the fibers is derived.Then a series of experiments on measuring the extinction ratio of the output light by changing temperature rapidly and using several light sources with different degree of polarization (DOG) were performed,and the above experimental results demonstrate that the light sources with low DOG are useful to reduce the mode-coupling effect in SM fiber coupler caused by temperature change.Moreover,the output errors of fiber optic gyroscope (FOG) owing to the mode-coupling effect were calculated and simulated with changing the extinction ratios of light sources,which is helpful on how to choose suitable light sources for different FOG systems.