An Er-doped fiber ring laser was characterized in order to examine the main causes that affects the linewidth of longitudinal mode beat(LMB) frequency. The frequency linewidth of LMB determines the resolution of the sensors using LMB signal. For example, the possibility of rotation rate sensing in Er-doped fiber ring laser was demonstrated recently, which used the LMB signal between clockwise(CW) and counterclockwise(CCW) laser outputs. The ring laser was constructed with 5m long, 500 ppm Er-doped fiber and a length of single mode fiber at the 1.55 ㎛ wavelength and polished type tunable directional couplers. It was pumped with an $Ar^+$ laser at the 514.5 nm wavelength and its lasing center wavelength was 1.53 ㎛. The ring laser oscillated in both CW and CCW direction with a stable output power. As the output coupling ratio varied from 0.05 to 0.8, its threshold pump power varied from 70 mW to 160 mW, and slope efficiency ranged from 5.5 × $10^{-5}$ to 1.44 × $10^{-3}$ for CW laser and from 1.5 × $10^{-4}$ to 6.89 × $10^{-3}$ for CCW laser, which was due to the location of the output coupler relative to the gain medium and the loss elements inside the cavity. The optical spectra and the rf beat spectra of both CW and CCW laser were the same. Since the laser operated in multiple longitudinal modes, the LMB signal contains many beat components of laser longitudinal modes within it. The frequency linewidth of LMB was about $10^{-3}$ times that of longitudinal mode spacing and showed a tendency that it increased as the pump power increased or the output coupling ratio increased. As the cavity length was increased the frequency linewidth of LMB decreased but the ratio of the frequency linewidth of LMB signal to the mode spacing increased. For the possible origin of the frequency linewidth of LMB and its behavior, nonlinear mode pulling effect and the effect of the dispersion of both Er-doped and undoped fiber were considered. But the magnitude of these effects were $10^{-4}$ and $10^{-3}$ times smaller than that of the experimental results, respectively. The dominant cause of the broadening of the frequency linewidth of LMB signal has yet to be discovered.
