Abstract: The experimental study of a passive mode locked Yb3+ doped fiber laser was reported, which utilized a 8 shaped ring cavity with semiconductor saturable absorber mirror(SESAM) as well as the polarizer and polarization controllers. Combining the effect of non-linear polarization rotation with the self starting of SESAM, stable operations of Q switched and CW mode locked regime at the low pump power were achieved. When the pump power was 18 mW, the Q switched pulse with a repetition of 16 kHz, a pulse width of 4 μs ,and an output spectrum width of 2.34 nm was obtained. When the pump power was 60 mW, the output power of mode locked pulses was 8 mW, the repetition was 20 MHz, the output spectrum width was 3.54 nm, and the pulse width was ps or sub ps orders. The tunable phenomenon with proper