This paper investigates the attitude synchronization problem of multiple spacecraft without a prescribed reference attitude in the presence of disturbance. Autonomous attitude synchronization is necessary when a spacecraft formation has no external reference information. Therefore, a decentralized robust attitude control scheme is proposed to guarantee spacecraft attitude synchronization under external disturbances in the absence of a common reference attitude or any other hierarchy in formation. The proposed class of controllers is effective with a large range of communication topological structure, and is designed by using the absolute angular velocity and absolute attitude quaternion. The control scheme is robust against environmental disturbance torques. The stability the resulting closed-loop system under the controllers are proven theoretically. Numerical simulations are included to validate the analytical results.