We present a multiwavelength mode-locked fiber ring laser incorporating two semiconductor optical amplifiers (SOAs) and a fiber Mach-Zehnder interferometer (MZI). One SOA is modulated by an injected external optical signal act as loss modulator, another SOA is not modulated by injected signal but services as cavity gain medium, and a fiber MZI functions as a tunable comb filter. Twelve synchronized wavelength channels with each channel of mode-locked at 10 Gb/s and pulse width of about 30 ps are achieved from the presented multiwavelength laser source. Oscillation wavelengths can be smoothly tuned by adjusting temperature of fiber MZI. Also, by adjusting the current of the SOA, oscillation wavelengths can be freely tuned in a large wavelength range. The output power is rather stable. This fiber laser has potential applications in longer waveband (L-band) within the low-attenuation window.