This paper addresses the problem of the robustness of a Luenberger observer applied to a given large-scale system to obtain the state estimation from the viewpoint of decentralized observation. While the large-scale system suffers parametric perturbations, a robust stability criterion for decentralized state estimation and control is derived. If saturating actuators are employed to drive the large-scale system, a new stability criterion is also given for the stabilization of large-scale systems by decentralized Luenberger observer-based constrained control. Finally, the results from several simulations are provided for illustration.