Background and purpose: The performance of full-scale moving bed biofilm reactor (MBBR) was evaluated as an alternative for upgrading of Kish Island Mirmohana wastewater treatment plant. In this study activated sludge process upgrade to MBBR process and different operating parameters were compared. Materials and Methods: Effect of upgrading on different parameters such as organic loading rate (OLR), mixed liquor suspended solid (MLSS), mixed liquor volatile suspended solid, sludge retention time (SRT), sludge volume index (SVI), hydraulic loading rate (HLR), also removal efficiency of chemical oxygen demand, biochemical oxygen demand and total suspended solid were investigated. Results: The study results show that with increasing the average flow influent (625.97 ± 38.6-1335.3 ± 102.06 m3/d) and reducing of aeration tank volume (300-150 m3), OLR (0.29 ± 0.1-1.82 ± 0.15), MLSS (1291.14 ± 463.43 mg/L-7382.85 ± 272.42 mg/L), SRT (12.5 ± 4.2 d-28.79 ± 3.84 d), SVI (54.94 ± 15.82-51.2 ± 9.31), HLR (13.85 ± 0.85-29.45 ± 2.25 m/d), and hydraulic retention time (4.61 ± 0.27-2.17 ± 0.17 h) were changed. Effluent concentrations under this operation condition were below the guidelines for irrigation water. Conclusion: Hence, MBBR process is a good alternative for upgrading wastewater plants especially when they have inadequacy space or need modification that will require a large investment.