Efficiency of the electron beam interaction with one of the periodic structure (PS ) slow harmonics depends on its amplitude .One possibi e method of the amplitude increase in the region of an electron beam motion may be the difraotional radiation resonance connected with the radIating harmonic birth. The frequanoy f at which this effect is observed,is connected with the PS period 1 and electron velocity Ye by formula: p (2/fl/J! (1 ) JA= 7?3 where n = —1 , -2, —3 , . . . ; = Ye/C , 0 is light speed. In this case the increase of amplitudes of all difractional har— monies of this frequency is observed I I ] . The most interesting case is that , when two harmonics are radiated simul taneuosly and one of them is born and another is radiated at O. angle (two-wave regime ) [2 1 . The parameters of this regime can be found from formula: g -/.; 5,t/2t (coi6, (2) As follows from correlation of (1)—(2), that resonance two—wave regime can be observed at rather litle speeds, which are smaller, than speed of light C. But their use demands a quantitive evaluation of radiation intensity. The many-wave regimes are rather widely used in the relativistic strong-current electronics providing a high—efficient electron—wave interaction. But until now the radiation intensity calculation doesn't take into acount resonances in these regimes. Therefore (as first — step) in this paper an evaluation of radiation intensity in two—wave resonance regime of difractional radiation is carried out experimenta— ly.