Scientific balloons have been launched in Japan by ISAS/JAXA since 1965, and Prof. Hashimoto’s group has been developing a micro-gravity experiment system, called BOV (Balloon-based Operation Vehicle), which uses free-fall from a high-altitude balloon. Flight experiments of BOV were carried out in 2006 (BOV1) and in 2007 (BOV2), when a fine micro-gravity environment had been established successfully. To achieve a longer period of the micro-gravity, usage of an air-breathing engine to surmount air resistance and to apply gliding flight control for safety landing have been investigated and developed for the next plan. However, to conduct the gliding flight control, the BOV body has to know its own absolute attitude with an accuracy of several degrees. We are now developing an attitude determination package, named ADP. This paper describes the design and development summary of ADP with the results of the balloon flight test which was carried out in September 2008. On the same flight, a GPS experiment was conducted and the results from the GPS-based pointing direction estimation experiment are compared with the ADP’s results. By the experiment, nominal performance of some sensors in the real environment was confirmed and the results of attitude determination by ADP and the GPS are discussed in this paper. Additionally, progress of the preparations for the next flight test, which will be performed in May 2009 is also reported and discussed.