Correlation of parameter c of elementary crystal cell and half-width of diffraction maximums on X-ray photogram with the average size of gibbsite crystals in a diameter is deduced from experiments. Put the case that this correlation is caused by increase in form factors of gibbsite plate crystals in process of their average size increase. Presence of microstrain in the crystal lattice γ-Al(OH)₃, focused lengthways the crystallographic axis c is the reason of maximum spread at X-ray photogram of macro-crystalline gibbsite samples.