GPS attitude determination system (ADS) uses carrier phase measurements observed at multiple antennas, and it contributes especially to simplification of the GN&C sub-system. The major problem of GPS attitude determination is how to improve reliability and accuracy. In particular, to establish a reliable attitude determination algorithm, which resolves integer ambiguity of carrier wave length, is the most critical subject. A usual integer search resolution is vulnerable to measurement error source with only one epoch data of a single carrier frequency, and also requires a high performance on-board computer. This paper presents the preliminary design result of a reliable and accurate algorithm determination. In the algorithm, the cost function to be evaluated for initial ambiguity search is accumulated double-difference (DD) measurement residuals in order to mitigate the effects of measurement errors. And it has a function of bias error estimation in single-difference(SD) equations to improve attitude accuracy. Besides, attitude propagation using Gyro rate data is adopted in order to reduce search space, i.e., computational load. This GPS attitude determination algorithm was verified by aircraft flight tests, which were conducted in November 2001. The accuracy was evaluated using IMU(Inertial Measurement Unit) data. Ambiguity resolution and attitude accuracy better than 0.5deg were obtained even during dynamic attitude maneuvers in a reasonable computation time.