Abstarct: This dissertation deals with designing decentralized fractional-order sliding mode controllers for large-scale nonlinear systems in the presence of uncertainties and interconnections. The proposed controllers are developed for both fractional-order and integer-order large-scale systems. First, two fractional-order large-scale systems with distinctive fractional derivatives are governed by defining two new sliding surfaces with corresponding robust control laws. Also, a multi-machine power system is considered as an integer-order large-scale system, and a robust power system stabilizer with a novel nonlinear sliding surface is suggested. In addition, to keep the synchronism and voltage level of power system, an interesting control scheme is utilized. All of the mentioned controllers are designed under the assumption that the uncertainties and interconnections upper bound is known. However, determining the upper bound in a large-scale system is troublesome. Then in the next step, an adaptive-fuzzy approximator is applied to fix this problem. Since the fuzzy approximator uses the adjacent subsystem variables as its own input, this strategy is known as semi-decentralized fractional-order sliding mode control. For all of the proposed controllers, stability of closed-loop system have been analyzed depend on the sliding surface dynamics by integer-order or fractional-order stability theorems. Finally, comprehensive simulations are carried out to show the effectiveness of the suggested controllers.