A stable narrow line-width dual-wavelength fiber laser based on nonlinear polarization rotation with a high finesse ring filter is proposed and demonstrated. A polarization-dependent-isolator (PDI) and a section of polarization maintaining fiber (PMF) form an equivalent birefringent filter. Moreover, a ring filter can produce high finesse due to the weak gain generated by the EDF. As a result, two stable wavelengths are generated which both have about 18dB extinction ratio, 16dB side mode suppression ratio (SMSR) and 0.16nm 3dB line-width. Plus, less than 2 pm of the wavelengths shift and smaller than 0.4 dB of the optical power fluctuation when the system is operating in room environment for a period of 40 minutes indicate that the ultra narrow line-width fiber laser is very stable.