We demonstrate a single-mode 6 cm long, phosphate fiber laser with maximum power of 9.0 W. Laser action output power per fiber length of 150 W m−1 was achieved and this is the highest value ever reported in a single-mode fiber laser using a phosphate glass fiber. The slope efficiency of the laser was 36.2% and lasing wavelength was 1028 nm. We used a 6% mol ytterbium-doped, air-clad photonic crystal fiber with the core of 30 µm in diameter. Estimated pump absorption in the fiber exceeded 400 dB m−1.