Nanocrystalline powders of GaN and SiC with grain sizes ranging from 2 to 30 ran were examined under high external pressures by in situ diffraction techniques. The experiments, which simulated the process of densification of pure powders under high pressures, were performed without a pressure medium, under so-called isostatic conditions. Mechanism of generation of internal strains and their distribution in nanoparticles was deduced from the positions and broadenings of Bragg reflections measured in situ under high pressures at room temperature: (i) macro-strain was calculated from average compressibility of the crystal lattice, (ii) micro-strain from FWHM of Bragg lines. Under external stress there are observed several pressure ranges in which micro- and macro-strains behave differently. Densification of nanopowders occurs in steps and it involves different mechanisms in relatively soft GaN (bulk modulus approximate to 125 [1]; approximate to 190 GPa [2]) and very hard SiC (approximate to 240 GPa [3]). The shapes of plots presenting generation of micro- and macro-strains change with the size of nanograins. Here we discuss the effects of size and agglomeration of powders on mechanism of relaxation of local strains under increasing external stresses.