A ring-shaped all fiber tunable Yb-doped fiber laser with tuning range over 20 nm is demonstrated by using a fiber Mach-Zenhder interferometer as an intra-cavity filter which is constructed with two 3-dB optical couplers. The method for fabricating the fiber Mach-Zenhder interferometer is detailed. The fiber laser has a moderate milli-Watt level output power over the whole tuning range from 1050 to 1071 nm,a side-band suppression ratio greater than 45 dB,and a bandwidth less than 0.1 nm.