Recent progress in GaN-based power electron devices and advanced thermal management techniques for high performance GaN power devices are described. First, recent advances in high performance GaN-based transistors are reviewed. Reduction of the fabrication cost is enabled by epitaxial growth technology on large area Si substrates. Normally-off operation has been achieved in our novel device structure called Gate Injection Transistors (GIT), which enables it with low enough specific on-state resistance and high drain current. Although the presented GaN power devices are promising for future emerging applications demonstrating high enough potential, power concentration due to tremendous chip size reduction requires superior thermal conductivity. We demonstrate the reduction of junction temperatures in low-pressure direct liquid cooling (LP-DLC) of presented GaN power devices for high-power and high-voltage applications. In order to achieve sufficient lifetime and high-voltage operation, LP-DLC using perfluorocarbon and perfluoroether as a working fluid are demonstrated for the first time.