There is considered theoretically the competition near the resonant acoustooptical (AO) conditions of two mechanisms which influence to the diffraction efficiency lid variation, when the thickness L of the sample (the length of interaction) is changed : 1) the enhancement of i due to increase of L, and 2) degrathtion of lid due to the increase of optical absorption q1 in crystal. It is shown that: 1) when changing the length of interaction L the maximum diffraction efficiency iid is changed and shifted in wave length's scale, reaching the maximum value at defined L which depend on the particular interaction geometry; 2) for particular considered in present work crystals the variation ofL is more critical to the wave length A change for GaAs as compared to InP; 3) when realizing the optimized geometry of AO interaction closed to resonant condition at frequency 2.5 GHz, the diffraction efficiency in GaAs crystals can reach 8% and in JnP 3.5 % per 1 mW of conducted electromagnetic power (conespondingly 8000 and 3500 % per 1 W of RF power) when the thickness of samples is about 4 mm.