We investigate the influence of fin architecture on linearity characteristics of AlGaN/GaNFinFET. It is found that the FinFET with scaled fin dimensions exhibits much flatter Gm characteristics than the one with long fins as well as planar HEMT. According to the comparative study, we provide direct proof that source resistance rather than tri-gate structure itself dominates the Gm behavior. Furthermore, power measurements show that the optimized FinFET is capable of delivering a much higher output power density along with significant improvement in linearity characteristics than conventional planar HEMT. This study also highlights the importance of fin design in GaN-based FinFET for microwave power application, especially high-linearity applications.