The production of cellulose-II nanocrystals (CNC II) requires a polymorphic conversion of natural cellulose-I raw materials. The transformation is usually achieved above a process like mercerization or dissolution-regeneration. This study demonstrated a new method for the production of CNC II directly from bleached kraft pulp (BKP, a commercially available cellulose I feedstock) in a weakly acidic lithium bromide trihydrate (MALBTH) system, a concentrated (∼61 wt%) Solution of LiBr in water with a very low concentration (2.5 mM) of sulfuric acid. First, the BKP was treated in the MALBTH system to produce a solid residue of cellulose II hydrolysis (CHR) with a yield of 64-86%, with the selective hydrolysis of the disordered cellulose and the polymorphic transformation being completed at the same time. Subsequent oxidation of the CHR by ammonium persulfate (APS, 0.1–0.6 M) led to CNC II with high yield (up to 62%), high crystallinity (over 90%), rich in surface carboxyl groups (0.3–1.2 mmol.) G-1 Cellulose), excellent colloidal stability (up to -59 mV zeta potential) and high thermal stability. The CNC II had an adjustable length (26–57 nm), determined by the conditions of the MALBTH hydrolysis and the APS oxidation, but a similar lateral dimension (8–10 nm). The characterization of the CHR by wide-angle X-ray diffraction and Fourier transform infrared spectroscopy confirmed the polymorphic conversion of cellulose I to II during the MALBTH treatment. The swelling of the BKP in the MALBTH enabled the cellulose crystallites to slide and reassemble, completing the rearrangement of the cellulose chains from the parallel to the antiparallel conformation (polymorphic transformation of cellulose I to II). This study provided an efficient and environmentally friendly method for the production of cellulose-II nanocrystals with controllable aspect ratios above the simultaneous hydrolysis and polymorphic conversion of cellulose I feedstocks.