This study investigated the impact of cellulose polymorphs on cellulose nanocrystals (CNCs) preparation. Cellulose samples with different types of polymorphs (cellulose I, cellulose II, and cellulose III) were prepared. Subsequently, CNCs were obtained from the above cellulose samples by sulfuric acid hydrolysis. The initial cellulose and CNCs were characterized by X-ray diffraction (XRD), thermo gravimetric analysis (TGA), scanning electronic microscopy (SEM) and particle size distribution (PSD). A comparative study showed that the properties of CNCs were closely related to the polymorphs of initial cellulose. The original polymorphs of cellulose I and cellulose II were retained, while that of cellulose III was converted back to cellulose I during the sulfuric acid hydrolysis process of CNCs preparation. For cellulose I and cellulose II, the crystallinity of both corresponding CNCs continuously increased with the increase of sulfuric acid concentration, until reaching a maximum at approximately 84.0%, while the crystallinity of CNCs obtained from cellulose III was slightly affected by sulfuric acid concentration. The yields of CNCs obtained from cellulose II and cellulose III with 61 wt% H₂SO₄ were extremely high, reaching 46.8% and 43.4%, respectively. Smaller CNCs particles with short-length were obtained by sulfuric acid hydrolysis of cellulose II. In addition, the properties (yield, dimension and thermal degradation behavior) of CNCs obtained from cellulose III were more susceptible to the acid concentration compared with those of CNCs obtained from cellulose I.