This paper addresses the agile attitude stabilization maneuver control of flexible-spacecraft using control moment gyros (CMGs) in the absence of modal information. Here, piezoelectric actuators are employed to actively suppress the vibration of flexible appendages. Both the attitude dynamics and the proposed robust controller are globally developed on the Special Orthogonal Group SO(3), avoiding ambiguities and singularities associated with other attitude representations. More specifically, an observer is first designed to estimate the modal information of vibration. A robust control law is developed by synthesizing a proportional derivative (PD) controller, an adaptive sliding mode controller, and an active vibration-suppression controller, which use the information of the estimated structural modes. The stability of the closed-loop system is proved using Lyapunov stability theory. Finally, numerical examples are performed to show the effectiveness of the proposed method.