A simple and effective tunable fiber laser is presented. The linear-cavity laser consists of a three-port optical circulator (OC), a tunable fiber Bragg grating (TFBG), a segment of high-concentration erbium doped fiber (EDF), and a wavelength division multiplexing (WDM) coupler. By using an OC as the total-reflection mirror and a strain-induced uniform FBG as a partial-reflection mirror and a wavelength selector, a tunable wavelength is obtained. With single TFBG, a single wavelength lasing with a maximum wavelength tuning range of 6.00 nm, from 1543.50 to 1549.50 nm, is achieved experimentally. Within this tuning range, the full-width at half maximum (FWHM) of the laser output is smaller than 0.01 nm and the side mode suppression ratio (SMSR) is better than 50 dB.