Dispersion-interaction corrected DFT simulations are performed to study the structure of two allomorphs of native cellulose I. Good agreement between theoretical and experimental data is achieved. Two H-bond patterns, previously identified experimentally, are examined for both allomorphs. The transition mechanism for the conversion between the phase I(α) and I(β) is studied by means of constrained relaxations. New metastable intermediate phase occurring on the I(α) → I(β) route is identified, and the corresponding structural data are reported.