Grain size and strain distribution functions of polycrystals of SiC with nanosize grains were examined based on X-ray diffraction data and ab initio calculations of scattered intensity from Debye functions. A tentative model of distribution of strain induced under high isostatic pressure in nanoparticles with different grain size is presented. Nanocrystalline SiC powders with grains down to 80Å in diameter were examined. In situ high pressure diffraction experiments were performed in cubic anvil cell MAX80 (up to 6 GPa) and in Diamond Anvil Cell (DAC) (up to 45 GPa) at HASYLAB, Hamburg, Germany. Shape of the Bragg lines was analysed with the use of two methods: (i) calculation of theoretical diffraction patterns based on modeling of one-dimensional disordering and ab initio calculation of scattered intensity starting from Debye functions and, (ii) approximation of the experimental shape of Bragg reflections by a combination of two functions: Gaussian (G) and Lorentzian (L).