Nanocrystals growth mechanism embedded into mesoporous thin films has been determined directly from grazing incidence X-ray diffraction data. We have shown, for the first time, that surface capillary forces control the growth mechanism of nanocrystals into these nanoarchitectures. Moreover, these data allow an estimation of the surface tension of the nanocrystals organized into a 3-D nanoarchitecture. The analysis of the variations in the strain field of these nanocrystals gives information on the evolution of the microstructure of these mesoporous films, that is, the contacts among nanocrystals. This work represents the first application of grazing incidence X-ray for understanding stability and performances of mesoporous thin films. This approach can be used to understand the structural stability of these nanoarchitectures at high temperature.