Modeling concrete at lower length scale is useful to predict the failure processes taking place in the material. In the present work, the deformation and failure of concrete is studied using the unit cell model. A unit cell simulates the deformation and failure in a representative volume element and postulates the material response curve based the response of a unit cell. A parametric study has been conducted to study the effect of aggregate size on the overall behavior of concrete. Uniaxial compression tests were simulated numerically. Interfacial transition zone (ITZ) was modeled using plane strain elements with brittle cracking model. Nonlinear stress-strain curves for various aggregate sizes were obtained using current unit cell approach. Simulation results indicate that the ITZ does not play a major role in determining the overall failure strength of concrete. Copyright © Taylor & Francis Group, LLC.