Abstract: Gallium nitride(GaN) monolithic integrated terahertz frequency multipliers are a promising solution to the power bottleneck in the next-generation terahertz systems. In the article, the architecture of GaN monolithic integrated terahertz frequency multipliers, the fabrication process of terahertz schottky barrier diodes (SBD), and the design of frequency multiplier chips have been investigated. The multi-anode GaN SBD topology and multi-section waveguide have been adopted to improve the power-handling capability and efficiency, thereby enhancing the output power. Three GaN monolithic integrated terahertz frequency multiplier prototypes operating at 170 GHz, 220 GHz and 340 GHz have been developed and tested, with measured results demonstrating superior performance. This technology exhibits great potential for applications including high-resolution imaging, ultra-high-speed converged communication, and high-precision remote sensing.