A 260 GHz frequency tripler based on GaN Schottky barrier diode is proposed. Unbalanced structure is adopted with GaN SBD chip to improve the tolerable power and output power of the circuit. The height-reduced and width-reduced output waveguide structure is employed to suppress the second harmonic. The input and output filter of the frequency multiplier is designed with high and low impedance strip line structure. The test results show that the frequency multiplier achieves a maximum output power of 69.1 mW and conversion efficiency of 3.3% at 261 GHz with good harmonic suppression characteristics.