A single-mode terahertz quantum cascade laser with metallic surface gratings is fabricated in order to obtain a terahertz source with narrow line width. Strong coupling efficiency and low waveguide loss are realized. A semi-insulating surface-plasmon waveguide is used to present a high output power. The lasing wavelength of this laser is 95 μm. The highest optical power of 43 mW and Side-Mode Suppression Ratio(SMSR) of 18 dB can be obtained under 10 K. Under the highest operation temperature of 70 K, an output power of 5 mW is still achieved. This kind of laser is the most promising solid-state source as the Local Oscillator(LO) for a high resolution terahertz heterodyne receiver.