Observer-based singular perturbation robust control is designed for a flexible-joint free-flying space manipulator with unknown payload parameters and bounded disturbances.In this robust control only the base attitude, the joint displacements and the joint elastic torques are assumed to be measured.The dynamic model of a free-flying space manipulator with two flexible revolute joints is established by applying the Lagrange equations.A flexibility compensator is proposed to make the equivalent joint stiffness large enough to make traditional singular perturbation approach applicable.