A stable C- and L-band tunable fiber ring laser, using a two-taper Mach–Zehnder interferometer (MZI) as a filter, is proposed and demonstrated experimentally. One of the two taper waists is mechanically bent to tune the laser wavelength. Being amplified by an L-band erbium-doped fiber amplifier and an erbium-doped fiber, respectively, the fiber ring laser has a full L-band ( $1564–1605\mathrm{nm}$ ) and C-band ( $1550–1565\mathrm{nm}$ ) tuning range with a side-mode suppression ratio as high as $50\mathrm{dB}$ . The laser linewidth and the minimum tuning step are related to the MZI’s cavity length. It was also found that thermal annealing relieved the internal stresses of the tapers and greatly improved the laser performance.