The use of cellulose nanocrystals as reinforcement in polymers brings challenges mainly related to the dry thermal processing, and their adhesion and dispersion in the matrix. In this work, cellulose nanocrystals from sugarcane bagasse was used as reinforcement in nanocomposites of acrylonitrile-butadiene-styrene. Thus, the main objective of this work is to evaluate the mechanical and thermal performance of cellulose nanocrystals/acrylonitrile-butadiene-styrene (CNC/ABS) nanocomposites as a function of cellulose nanocrystals type and content. More specifically, to study the effects of the chemical interaction between acrylonitrile groups from ABS and hydroxyl groups from cellulose. The ABS and the nanocomposites, reinforced with 0.5, 1.0 and 1.5 wt% nanocrystals, were obtained by extrusion and then injection molded to obtain specimens for testing. Mechanical testing, dynamic mechanical analysis, rheology and thermogravimetric analysis were used for characterization. The main results are related to increased impact strength, elastic modulus, storage modulus and viscosity of the nanocomposite compared to ABS. However, tensile strength and thermal stability decreased. Therefore, if higher tensile properties are needed, the CNC₂₂₀/ABS nanocomposites should be preferred whereas, for impact, CNC₁₅₀/ABS. Thus, there is a general improvement in properties up to 0.5% cellulose nanocrystals content, which is followed by a loss in properties for higher content.