Cellulose nanofibrils can be obtained from microcrystalline cellulose by acid hydrolysis processes. Under optimum hydrolysis conditions is possible to obtain cellulose nanofibers with high surface/volume ratio, high aspect ratio (length to diameter), high crystallinity and improved thermal stability. All these parameters then determine their effectiveness as reinforcement in a polymer matrix. In this work, cellulose nanofibrils were obtained from commercial microcellulose supplied by Aldrich. The acid hydrolysis synthesis was optimized studying the effect of reaction time and temperature and acid solution concentration. The optimized parameters were selected so as to obtain fibers with high crystallinity, high aspect ratio with diameter in nanoscale and high thermal stability. The morphology and size (length and diameter) of the fibers was analyzed by Field Emission Scanning Electron Microscopy (FESEM), the chemical structure by Fourier Transform Infrared Spectroscopy (FTIR), thermal stability by Thermogravimetric Analysis (TGA) and crystallinity by X-ray Diffraction (XRD). Copyright © VBRI Press.