The aim of this paper is to develop a model for simulating carbonation of fly ash concrete. The model describes the diffusion of gases (CO2 and water vapor) in concrete by the mass balance equations and the Fick’s law of diffusion. The time dependent hydration, and pozzolanic and carbonation reactions were considered for evaluating the amount of calcium hydroxide in concrete. The rate of carbonation reaction was formulated based on the Arrhenius’s formulation. The model was verified by using the results of relative water content, amount of calcium hydroxide, and carbonation depth tested in accelerated and real environments. The influence of changes in parameters on mix proportion and environmental condition were investigated. The verification results were found to be satisfactory in the prediction of carbonation of fly ash concrete