Tunable differential phase-shift keying (DPSK) demodulator based on fiber Mach-Zehnder interferometer (MZI) is investigated, which has advantages of low insertion loss, high isolation, good stability, and tunable phase. Operation principle for DPSK demodulation is analyzed. Temperature of the fiber arm is set by the temperature control system, in which the tuning range is 20 ℃ and the resolution is 0.1 ℃. In the experiment, 40 Gb/s DPSK signal was demodulated successfully, and the extinction of the demodulated signal is 14.5 dB. With the help of temperature control, the experimental results have a long-time stability and can be switched between constructive interference and destructive one.