This paper proposes a new method to design an observer for estimating the unmeasurable states variables of the large-scale nonlinear system. The large-scale nonlinear system in this study is represented in term of a set of polynomial T-S fuzzy subsystems with interconnection terms. The interconnection parts of this paper are arbitrary. The unknown input method is applied to design observer for both estimating unknown states and eliminating the effects of the interconnection terms. Based on the Lyapunov methodology and Sum-Of-Square (SOS) technique, the conditions for observer synthesis are derived in the main theorems. Finally, an illustrative example is provided to prove the success of the proposed method.