We report the first demonstration of fully-vertical power MOSFETs on 6.6-μm-thick GaN grown on a 6-inch Si substrate by metal-organic chemical vapor deposition (MOCVD). A robust fabrication method was developed based on a selective and local removal of the Si substrate as well as the resistive GaN buffer layers, followed by a conformal deposition of a 35-μm-thick copper layer on the backside by electroplating, which provides excellent mechanical stability and electrical contact to the drain terminal. The fabrication process of the gate trench was optimized, improving considerably the effective mobility at the p-GaN channel and the output current of the devices. High performance fully-vertical GaN-on-Si MOSFETs are presented, with low specific on-resistance (R on,sp ) of 5 mΩcm 2 and high off-state breakdown voltage (BV) of 520 V. Our results reveal a major step towards the realization of high performance GaN vertical power devices on cost-effective Si substrates.