A tunable ytterbium doped double cladding fiber laser with high efficiency has been investigated. The output power of the fiber laser was about 440 mW using 1 W 915 nm LD pumping and the slope efficiency was about 80%. The M 2 factor of the output beam in both directions were 1.06 and 1.04, respectively. The output wavelength can be tuned from 1070 nm～1150 nm while using a grating at output coupler.