In the context of the development of the next generation of nuclear reactors, SiCiSiC composites are candidate for structural applications. Because of their complex thermo-mechanical behaviour, due to their complex microstructure, a multi-scale approach is under development. An important microstructural parameter of the CVI composite is the complex distribution of the residual porosity inherent to the CVI process. This paper focuses on the characterization of the macroporosity (the porosity among the tows) and on its effect on the thermo-mechanical behaviour. The experimental characterization of the macroporosity is performed using an X-ray tom0 aphy technique on the beamline ID19 at the ESRF synchrotron (France) with a resolution of 5.023pm . The 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, called "statistical volume elements", with at least 2 plies per volume element have to be used in order to account for the variability of the stacking of the plies. Finally, such "statistical volume elements" (SVE) are directly extracted from the tomographic image and the "effective" elastic behaviour is evaluated from the average of the "ap arent" behaviour evaluated on each SVE. In spite of their quite important size (3.2x3.2x0.45mm ), the "apparent" behaviours evaluated for each SVE exhibit important fluctuations.