Abstract: A 3D attitude determination system is established. Theory and algorithm of attitude determination based GPS are addressed in this paper. Least square adjustment is used to calibrate baseline, then get baseline length and angle between two baselines, which can be used as a restriction to accelerate ambiguity resolution. Vector of baseline is estimated to get coarse attitude, ultimately, the precise attitude is estimated by iterative least square adjustment. All the discussed above are validated by attitude determination system.