There is a demand for attitude in mobile surveying systems. Thus, a direct attitude determination method based on vehicle-mounted three-antenna global navigation satellite system (GNSS) is proposed. Principles behind attitude determination were analyzed and the formulae for the folution were derived. A simple method is proposed to determine the quadrants of the yaw angles, which solves the multi-value problem. The data from a land vehicle are processed, and a reference attitude from a high-accuracy inertial navigation system (INS) was used to evaluate the accuracy and reliability of the attitude determined from three-antenna GNSS. The results indicate that the attitude determined from three-antenna GNSS can achieve high accuracy and reliability. This method is simple in calculation and avoids the singularity problem.