Abstarct: This tesis introduces an optimized fuzzy controller by the PSO algorithm using a nine-level diode-clamped inverters for direct control of power injection into the grid. The conventional control method in grid-connected multi-level inverters is to control output current, whereas in this work, the active and reactive desired powers are directly used as a tracking reference values. In comparison with the integral controller, the proposed fuzzy control approach has significant advantages such as eliminating steady-state error and decreasing the oscillation amplitude in the injected powers. Improving power quality, harmonics reduction and efficiency enhancement are also some of additional superiority of multi-level inverters than two-level ones. Moreover the performance of the proposed controller using five-level diode-clamped inverter was investigated. Finally, the simulation results of the two control strategy and invertes are compared with each other that confirms the effectiveness of the proposed system and power injection control approach.