An airborne attitude determination system using GPS phase interferometry has been developed by National Aerospace Laboratory (NAL) and flight tested. Two GPS antennas are mounted 6.41 meters apart on the fuselage of the aircraft so that the pitch and yaw angles can be determined. An INS (Inertial Navigation System) is also installed to provide an independent attitude reference. Results of post-flight analyses are presented showing that the attitude estimated by GPS agrees with that of INS within 0.1 degrees in pitch and 0.2 degrees in yaw angle.