Abstract: Presents a new experimental result of an extreme narrow spectral line-width of single mode output obtained from an Er-doped optical fiber laser. Compress on of line-width by compound ring structure is theoretically analyzed. The influence of active compound ring structure parameters on the extreme line-width is discussed. Experimental results demonstrated that the linewidth of the single longitudinal mode output was less than 100 Hz. The maximum output power is 2.779 mW.