Thermally expanded core fibers were made by heating a singlemode fiber locally with hydrogenoxygen flame. An all fiber MZ filter was fabricated using the thermally expanded core fiber, and it was inserted into a Sagnac filter to make a new structure of tunable fiber Sagnac filter which output spectrum was the uncorrelated superposition between the Sagnac and the MZ interferences. With the new structure Sagnac fiber filter, a tunable erbiumdoped fiber laser was made and investigated experimentally. Tunable output was obtained by adjusting a polarization controller (PC) in the Sagnac interferometer. During the whole tuning process, the laser output wavelength could be tuned from 1 540.3 nm to 1 581.2 nm with the output power flatness better than 2 dB, 3 dB spectral linewidth narrower than 0.1 nm and optical signaltonoise extinction ratio higher than 45 dB.