Scientific group has conducted the most precise experimental determination of ratio between mass and energy. To check world famous equation researchers used device GAMS4, created in NIST, but located in French Institute Laue-Langevin at present moment. Experiment was based on exact angle measuring of gamma rays` diffraction by 2 identical crystals with known lattice distance. Using this device scientists obtained most precise energy values, emitting by one silicon or sulfur atom during capture of neutron. They also used precise measurements of atom masses. Finally, they were able to correlate defect of mass during neutron capture by atom with energy of gamma quantum, emitted in the same capture. The authors of the experiment have established that energy E differs from mc2 less than 4-th part from 10 millions. This estimate is 55 times more precise than previous direct test of famous Einstein`s formula.