With the application demands of solid-state high power in the terahertz (THz) band, a THz-band watt-level power output is achieved by adopting GaN power amplifier (PA) MMIC and power combining technology. Microstrip-waveguide transition, and the low-loss interconnection based on the gold wire compensation are used to package a PA module composed of two PA MMICs and an E-plane T-junction two-way power splitter/combiner. Maximum output power of 160 mW is achieved. Based on the module and an eight-way E-plane combiner, a sixteen-way power combining amplifier is designed across the frequency range of 180 to 238 GHz. Output power of more than 300 mW is achieved with +10 V drain voltage, and the maximum power is 1.03 W at 189 GHz.