The solid phase reaction of yttrium thin film and silicon substrate was investigated in situ with help of RHEED technique. For the first time, we have measured directly yttrium silicide formation temperature which can be as low as 120°C for a thin (60 ) metal layer deposited on a Si(111) substrate. For this purpose, we developed a new experimental technique. A study of the growth of thin (10-150 ) silicon overlayers on yttrium and dysprosium silicide films epitaxially grown on Si(111) was also made. In this paper, an in situ reflection high energy electron diffraction (RHEED) pattern and also azimuthal plot investigation of Si/RE-silicide/Si double heterostructures grown by solid phase epitaxy and reactive deposition method are presented.