This paper focuses on the characterization of the macroporosity, the porosity among the tows, observed in chemical vapor infiltration composites and on its effect on the thermo-mechanical behavior. The experimental characterization of macroporosity is performed using an X-ray tomography technique. Numerical 3D images are used to describe the distribution of macroporosity with respect to the position of the plies. It is clearly established that the stacking of the plies has a significant effect on the porosity distribution. As a consequence for the micromechanical modelling, a unique element that contains only one ply is not representative of the porosity distribution and is not sufficient to evaluate the "effective" mechanical properties; several volume elements (VE), called "statistical volume elements (SVE)," with at least two plies per VE have to be used in order to account for the variability of the stacking of the plies. Finally, such SVE are directly extracted from the tomographic image and the "effective" elastic behavior is evaluated from the average of the "apparent" behavior evaluated on each SVE. In spite of their quite important size, the "apparent" behaviors evaluated for each SVE exhibit important fluctuations.