A new technique is suggested in order to find the limiting thickness of an epitaxial Si layer h(epi). It is based on monitoring the intensity variations of a bulk spot in the RHEED pattern during MBE growth. A maximum is observed in the plot of intensity versus deposited film thickness. The maximum intensity corresponds to h(epi). The activation energy of the limiting thickness epitaxy is 0.5 eV. Thus, the process of low-temperature growth and the surface recovery can be controlled in situ by monitoring of variations in intensity of diffraction peaks. To achieve the control, registration of intensity Variations of not only bulk spots but also superstructural ones should be used.