In this paper phase state and phase transitions of nano-scale cellulose crystallites have been studied. It was found that conditions of the phase transitions of rod-like nanocrystallites are depended on their lateral size (Lcr). Small nanocrystallites have instable phase state; as a result they have tendency to co-crystallization with forming of larger crystallites having a low specific surface. After treatment in boiling water small nanocrystallites of CIII easily recrystallize into crystallites of CI, whereas for CIII having large crystallites this phase transition cannot be implemented. It is also found that after heating in glycerol at 533 K three allomorphs, CI, CII and CIII, can be transformed into CIV, when the lateral size of nanocrystallites Lcr ≤ 4.3 nm; whereas the larger crystallites of these allomorphs cannot be converted directly into CIV. Furthermore, small nanocrystallites of CI are more soluble, and their transformation into CII occurs at lower concentrations of mercerizing agents.