In view of the importance of aggregate density to the mechanical properties of concrete, it is necessary to determine the aggregate density at any point in concrete or any cross-section of a concrete element. Once the aggregate density is known, some other mechanical properties of the concrete can be determined based on the theory of micro-mechanics. This paper attempts to simulate aggregate distribution in plain concrete taking into account of wall effect. Based on the study of the characteristics of the simulated aggregate distributions, a model of aggregate density is developed and its statistics are derived. Simulation results show that the average aggregate density of concrete in both two and three dimensions converges asymptotically to a constant when the number of simulations tends to infinity. Using mathematical regression, the aggregate density as a function of distance from the boundary and the thickness of the boundary layers can be expressed analytically in terms of the aggregate area or volume fraction of the concrete. The experimental verification of the developed model of aggregate density is also presented in the paper.