Cellulose nanocrystal (CNC) has recently gained much attention due to its unique properties such as abundancy, biodegradability, high strength, large surface area, functional ability, template structure, and sustainability. To broaden its application and enhance its compatibility with other materials, CNC can be modified via different methods. The modification is based on introducing new functions, including esterification, silylation, carbamation, polymerization, and so on. The application can concern many fields, such as polymer reinforcement, packaging, water treatment, textiles, biosensors, etc. Herein, we summarize the main approaches employed for the chemical modification and the use of the modified CNC material in the preparation of nanocomposite films and membranes, along with some emerging applications.