GaN-on-diamond high electron mobility transistors (GaN-on-diamond HEMTs, GoD HEMTs) were successfully fabricated using a large size mosaic diamond substrate. A mosaic diamond with size of 15mm×15mm was prepared and finely polished using novel polishing technique utilizing hydroxyl radicals generated through the catalytic reactions between transition-metal and hydrogen peroxide. GaN-based thin film which contains HEMT patterns were separated from the original substrate and finely polished. Both polished surfaces were bonded using surface-activated room temperature bonding technique with inserting very thin interfacial layer. Measured current-voltage characteristics and temperature imaging results showed that heat transition through the substrate in the fabricated GoD-HEMT improved drastically compared to the HEMT device fabricated using GaN-based thin films grown on Si substrate.