In this study cellulose nanocrystals (CNC) were prepared by sulfuric acid hydrolysis from microcrystalline cellulose with average length of 143 ± 71 nm and average width of 11 ± 4 nm, as observed in field emission scanning electron microscopy (FE-SEM). The CNC were submitted to an “in situ” polymerization treatment to graft poly(butylene glutarate) of low molecular weight on their surface. The grafting was confirmed by infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The crystalline structure of cellulose type I was preserved, confirming chemical modification only on the surface, as verified by wide angle x-ray diffraction (XRD). Thermogravimetric analysis (TGA) showed that the chemical treatment increased the thermal resistance of CNC. Composites of biodegradable poly(butylene adipate-co-terephthalate) – PBAT – with 10 wt% of modified and non-modified CNC were prepared through melt extrusion. CNC addition led to an increase of 50% in the elastic modulus of PBAT. In addition, dynamic mechanical thermal analysis (DMTA) showed that the composite with CNC retained its high modulus even at high temperatures (60°C), far above the glass transition temperature of PBAT (-30°C), enlarging the possibility of applications of this polymer. Therefore, the results of this work showed that cellulose nanocrystals have potential to be used as reinforcements in PBATmatrix and that this field must be explored.