This letter proposes a high output power 250-GHz power amplifier adopting dual-shunt elements in the implementation of maximum achievable gain (G(max)) core. By the adoption of the dual-shunt-element-based G(max)-core, the output transistor size can be increased, which leads to higher output power. Implemented in a 65-nm CMOS, the measurement results show a peak gain of 12.6 dB, P-sat of 3/3.8 dBm, OP1 dB of 1.4/2.3 dBm, and peak power-added efficiency (PAE) of 4.8/3.2% at 248.6 GHz while dissipating 40/62.4 mW from a 1/1.2-V supply, respectively.