Abstract: The utilization of recycled concrete aggregates (RCA) in a new concrete production has been increasing due to environmental and economic considerations. In this paper, concrete is considered as a random heterogeneous porous composite including three phases: natural or a recycled coarse aggregate, a porous matrix and an interfacial transition zone (ITZ) between them. Numerical samples for gas permeability analysis are established through a random aggregate structure (RAS) scheme, each numerical sample containing randomly distributed aggregates coated with ITZ and dispersed in a homogeneous matrix. Then the finite element method (FEM) is used to solve the steady permeation problem on a 2D numerical sample and the overall permeability is deduced from a flux-pressure relation. From numerical results, the following observations are obtained: 1) overall permeability increases non-linearly with the width and relative permeability of ITZ; 2) the dilution and tortuosity effects of aggregates on gas flow through the porous matrix are two predominant factors for permeability estimation; 3) the directional contribution of coarse aggregates on overall permeability shows an “S” shape curve behaviour. For high durability performance RCA with, it is preferred that the permeability should be at least 1 order lower than matrix. The replacing ratio of RCA should also be determined reasonably.