The effect of high hydrostatic pressure - high temperature treatment on strain state of thin GaAs layers, grown on vicinal (001) Si substrate, misoriented towards [110] with different miscut angles and with two different orientations of GaAs layer in relation to the miscut direction, was investigated by X-ray diffraction methods. The strain of the GaAs layers and GaAs/Si samples bending decreases when subjected to the treatment at T = 670 K under enhanced hydrostatic pressure (1.2 GPa). Dependence between the stress-induced structural changes and the orientation of GaAs miscut angle as well as peculiarities of the temperature-pressure treatment were determined. It has been shown that the amount of relaxed strain depends on the initial defect structure of the GaAs/Si hetrostructure. After reannealing the samples at higher temperature (870 K - 1.2 GPa), the strain and sample bending were the same as that after the treatment at 670 K - 1.2 GPa but the defect structure of layers was improved
