We developed a gas-sealing technique by adopting a metal capsule in a belt-type high-pressure apparatus, which enabled a supply of high-pressure nitrogen gas of several hundred MPa. With employing the sealing technique, cubic-GaN (c-GaN) nanoparticles were successfully synthesized using the Na flux method under nitrogen pressure above 100 MPa. A high cubic/hexagonal ratio of more than 20 can be achieved by optimizing the synthesis conditions. An attempt was made to functionalize c-GaN by doping Eu element as a luminous origin. Cathodoluminescence and XANES study revealed that Eu can be doped into the c-GaN particles, causing them to take on the state of Eu(3+). (C) 2011 Elsevier B.V. All rights reserved.