We demonstrate the first GaN vertical transistor on silicon, based on a 6.7-mu m-thick n-p-n heterostructure grown on 6-inch silicon substrate by metal organic chemical-vapor deposition. The devices consist of trench-gate quasi-vertical metal-oxide-semiconductor field-effect transistors with a 4-mu m-thick drift layer, exhibiting enhancement-mode operation with a threshold voltage of 6.3V and an ON/OFF ratio of over 10(8). A high OFF-state breakdown voltage of 645 V along with a specific ON-resistance of 6.8 m Omega.cm(2) were achieved thanks to the high-quality 4-mu m-thick GaN drift layer, presenting a relatively low defect density and very high electron mobility (720 cm(2)/V.s). This excellent performance represents a major step toward the realization of high-performance GaN vertical power transistors on low-cost silicon substrates.