The crystallinity index for native cellulose, CNCH2SO4 and CNCCr(NO3)3 were calculated based on Equation (5) and found to be 65.7%, 81.4% and 86.5%, respectively. It was noticeable that the produced CNCH2SO4 and CNCCr(NO3)3 exhibited higher crystallinity compared with starting material. This is because of the successive removal of the less-ordered surface cellulose chains in the matrix during the catalytic hydrolysis process, leading to the exposure of solid elementary crystalline cellulose phases . In this study, the untreated MCC was less crystalline than the treated ones. The higher intensity peak associated with CNCH2SO4 and CNCCr(NO3)3 was predominantly due to the successive dissolution of the defective cellulose regions with the consequent increase in the sample crystallinity index. Although H2SO4 and Cr(NO3)3 exhibited the strong catalytic effects toward the degradation of cellulose, the maximum crystallinity index was obtained with CNCCr(NO3)3 (86.5% ± 0.3%) compared to that of CNCH2SO4 (81.4% ± 0.1%). In the process of hydrolysis, the H3O+ ions tend to penetrate into the non-crystalline regions of the cellulose matrix, promoting the hydrolytic cleavage of glycosidic linkages and eventually releasing the individual crystallites. On the other hand, the higher crystallinity of nanocellulose product could be attained because the nanocellulose tends to be realigned into a better organized and crystalline structure, which results in the self-assembly phenomenon of nanocellulose, enabling close packing and hydrogen bond formation . Besides that, there is no remarkable difference in the XRD patterns between CNCH2SO4 and CNCCr(NO3)3. These results confirmed that the proposed Cr(NO3)3 hydrolysis system does not have a negative effect on the crystallites structure of cellulose.