Abstract: An environment-friendly method for obtaining cellulose nanocrystals (CNC) using deep eutectic solvents (DES) was developed. Formation of highly crystalline CNC with average particle dimensions 20 × 100 × 700 nm was confirmed with SEM and AFM. Molecular dynamics simulations demonstrated that the hydrogen bond interactions of the cellulose hydroxyl groups with the urea C=O group and with the chloride ions were the key factors of the destruction of MCC particles in the process of solvation. The type of cellulose crystal structure (I_β) and the high degree of crystallinity (about 80% according to Segal method) were preserved during treatment with DES. The ability of the prepared CNC to act as a reinforcing filler was tested by introduction of them into the chitosan-based films plasticized with DES. It was found that addition of 2 wt% of CNC led to an increase in the strength of the films from 11.4 up to 20.4 MPa with a simultaneous increase in the elongation at break. Graphic abstract: [Figure not available: see fulltext.].