Aeration is an important step in water and wastewater treatment processes, in which, microorganisms break down soluble and insoluble organic constituents and produce biomass. The excess biomass (sludge) produced each day is processed and removed and the treated effluent discharged out of the plant. Because of the low solubility of oxygen and the consequent low rate of oxygen transfer, required oxygen for the aerobic treatment does not enter the basin through normal surface air-liquid interface highlighting the need for aeration systems. Diffused aerators have been extensively developed and used since 1980s due to several advantages, most importantly high oxygenation performance. However, aeration process can account for up to 70% of total energy consumption of a wastewater treatment plant. Therefore, the proper design of aeration systems and optimizing oxygen transfer can reduce energy consumption and operation costs as well as guarantee a reliable and efficient treatment. Oxygen transfer efficiency depends on many geometric and dynamic factors such as geometry of the tank and diffusers, inlet air flow rate and bubble diameter. There are numerous empirical studies that predict the oxygen transfer process often with focusing on a certain number of parameters, while investigations show that effective parameters must be considered concurrently. Accordingly, conceptual design and process control of the aeration systems are still based on general empirical guidelines and operators' experience. With the development and expansion of the computational fluid dynamics tools (CFD), study of bubbly flows and oxygen mass transfer has become more convenient and accurate. However, due to over-simplifications and some restrictions in modeling of bubbles and their behavior in some studies, they look inadequate and incomplete. The current research is aimed at studying the effects of all parameters affecting oxygen transfer and thus proposing a correlation to predict oxygen transfer and design diffused aeration systems. In this research, the main objective of using CFD is to provide a comprehensive and accurate means for modeling bubble generation and dispersion based on a robust population balance model and modeling oxygen mass transfer based on the standard two-phase ‌mass transfer model known as two-film theory. Governing parameters in the oxygen transfer process were determined. We then used dimensional analysis to introduce dimensional groups. Afterwards, the laboratory observations and numerical results were analyzed based on image processing to obtain various flow and concentration contours. The contours of dissolved oxygen concentration indicated that the DO concentration is lower at the bottom of the tank. Accordingly, the effect of each parameter on oxygen mass transfer was measured and evaluated. The results revealed that increase in the air flow rate, diffuser submergence depth and diffuser diameter have positive effects on oxygen transfer coefficient. On the other side, decreasing the tank width and bubble diameter significantly increase oxygen transfer coefficient. Finally, obtained experimental and numerical data were fitted to reach a correlation predicting the oxygen mass transfer coefficient. The obtained correlation was evaluated by the measured experimental and numerical data. The correlation coefficient was calculated as 0.96 and most data were within the error range of ±20%. The proposed equation offers a practical means to design the diffused aeration systems and optimize the oxygen transfer. Keywords : Aeration; Oxygen transfer; Wastewater treatment; Diffuser aerator; Bubble