Cellulose nanocrystals were obtained from pressed oil palm mesocarp fibers using a chlorine-free pulping method. Raw fibers were isolated and alkaline-treated, followed by bleaching with peroxide. Subsequent hydrolysis of the bleached fibers provided the cellulose nanocrystals, which were characterized by transmission electron microscopy, Zeta potential, Fourier-transformed infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. Cellulose nanostructures had typical lengths (L) of 171 ± 15 nm and diameters (D) of ∼5 nm, yielding aspect ratios (L/D) as high as 35. The zeta potential of the nanostructures was –26.1 mV. The cellulose nanocrystals presented a typical infrared spectrum. The crystallinity index of cellulose nanocrystals was recorded as 70.9% (84% higher than the raw fiber) and the cellulose nanocrystals presented a good thermal stability. Thus, acid hydrolysis was effective for the extraction of cellulose nanocrystals and the physicochemical properties of the extracted cellulose nanocrystals indicate that they are suitable to reinforce polymer materials like thermoplastics, among other applications.