Iridescent films constructed by self-assembly of rod-like cellulose nanocrystals (CNCs) are mechanically brittle and of poor thermal stability. Herein, we demonstrate that the toughness and thermal stability of vacuum filtered CNC iridescent films could be significantly improved by a simple alkali treatment. The results reveal that the NaOH treatment can simultaneously alter the condensed physical structure and surface chemical structure of CNCs in their liquid crystal state while the self-assembled CNC chiral nematic structure still well retained. To be specific, the CNC was transformed from a higher crystallinity of form I to a lower crystallinity of form II, and the sulfate groups of CNCs were erased by alkali treatment, resulting in the remarkable enhancement in mechanical and thermal properties of CNC iridescent films. Of note, the unprecedented improvements in both tensile strength and toughness of CNC iridescent film have been achieved by alkali treatment. A sandwiched model with interdigitated mol...