Intermodulation effects play a decisive role in any system relying on the linear transmission of signals, such as highspeed wireless communication. For the first time, this paper presents measured third-order and fifth-order intermodulation performance of a 300GHz monolithic integrated circuit with a new photonic approach. Conventional amplifier characterization in this frequency range showed a small-signal gain of more than 15dB, operating frequency range from 279 to 313GHz, and a 1dB compression point and saturated output power of 1.6dBm and 6dBm, respectively, from a one-tone power sweep. Using a novel photonic-based measurement setup, the amplifiers' output-related IP3 and IP5 are measured to 13dBm and 11dBm, respectively. The two-tone measurement uses the photonic mixing of three laser lines to generate the two-beat frequencies at 299 and 301GHz.