A novel temperature sensor based on a dual-core fiber (DCF) is proposed and theoretically analyzed. The DCF-based temperature sensor is simply formed by splicing a segment of DCF to two segments of single mode fibers, where the DCF is used as the sensing element. The mode coupling between two fiber cores of the DCF is sensitive to the temperature-induced index change of the silica in the DCF. Simulations show that there is a linear relationship between the temperature of the DCF and the wavelength shift of the output spectrum of the DCF-based temperature sensor when the broadband light is injected into one fiber core of the DCF. Temperature sensors based on DCFs with different parameters for temperature sensing are also investigated.