Tunable fiber lasers with ring resonator structure were reported in this paper. A tunable fiber directional coupler links the rear-earth-doped single-mode fiber to from a ring resonator. Laser oscillation should occur in the fiber ring cavity when the rear-earth ions in the fiber are excited by a pumping light. The losses of the directional coupler and rear-earth-doped fiber are very small so that the fiber ring laser has low threshold and high slope efficiency. In this fiber ring laser, the directional coupler plays three functions of transmitting pumping light, coupling lasing oscillation and tuning laser wavelength. We have used both Nd3+-doped and Er3+-doped single-mode fiber to fabricate the tunable fiber ring lasers. An argon ion laser was used as the pump source. For the Nd3+-doped fiber ring lasers with cavity lengths of 3 meters and 5 meters, the lasing threshold were 0.8 mW and 4.9 mW respectively at the lasing wavelength of 1.088 nm. The highest slope efficiency was about 30%. The laser wavelength could be tuned continuously in the Nd3+ fluorescent spectrum and the tunning range is greater than 60 nm. The Er3+-doped fiber ring lasers had the cavity lengths of 1 meter and 8 meters, lasing at 1550 nm with a tuning range of about 70 nm, the thresholds were 1.9 mW and 14.2 mW and the slope efficiencies were 6.1% and 10.2% respectively. A novel fiber ring laser with single-longitudinal-mode output was demonstrated. This fiber laser has a composite resonator which is composed of two subrings. Longitudinal mode is selected in the subrings coupled each other. A laser linewidth less than 40 MHz was got from the Nd3+-doped fiber ring laser.