An asymmetrical π ‐phase‐shift distributed feedback (DFB) fiber laser written in erbium/ytterbium co‐doped fiber is fabricated. A new self‐injection locking method is employed to make the proposed DFB fiber laser operate in single polarization states, which is composed of a polarization‐dependent isolator and an optical circulator. The output character is investigated. 9.5 mW of the backward output power is obtained at the 159 mW pump power. The output power in one side is much higher than that in the other side, indicating that the DFB fiber laser operates in the unidirectional output. The delayed self‐homodyne measurement is employed to detect the radio frequency spectrum of the DFB fiber laser. This novel single polarization DFB fiber laser has the potential application in the fiber optical sensor and optical communications. (© 2010 by Astro Ltd., Published exclusively by WILEY‐VCH Verlag GmbH & Co. KGaA)