A Mach-Zehnder interferometer using a Twin-core Fiber(TCF) was researched for temperature sensing and a comb-filter based on the in-fiber Mach-Zehnder Interferometer (MZI) was fabricated by splicing a section of single mode TCF between two single mode fibers (SMFs). The Free Spectral Range(FSR) of the transmission spectrum for the device versa the wavelength, the length of TCF and the effective index difference between the two cores were analyzed and the temperature characteristic of the device was experimentally tested.The tested results indicate that the transmission spectrum of the device has a red shift and a quadratic curve with the temperature increasing. Under the same temperature difference, the wavelength shifts are different at different wavelengths and the wavelength shift at short wavelength is smaller than that at longer wavelength. The sensitivity of the device is different at different wavelengths, and it is proportional to the temperature at a fixed wavelength. Furthermore,the insertion loss of the device is decreased effectively by tapering the splicing regions between SMFs and TCF.Finally a device with insertion loss about 7 dB was implemented. This device is relatively small, simple, compatible and suitable for fiber optic temperature sensors.