Abstract: Nanocellulose is a sustainable resource that combines a modifiable surface with excellent mechanical strength. It has been increasingly applied to applications in different fields. Plant-based nanocellulose primarily contains cellulose nanocrystal and cellulose nanofibrils. This review summarizes the preparation methods for cellulose nanocrystal and cellulose nanofibrils that emerged from agricultural by-products. The different sorts of cellulose nanocrystal preparation methods that have been discussed include the inorganic acid hydrolysis method, organic acid hydrolysis method, ionic liquid method, deep eutectic solvent method, and so forth. Also, several methods of pretreatment and preparation of cellulose nanofibrils are characterized, encompassing the oxidation pretreatment represented by 2,2,6,6-tetramethylpiperidine-1-oxy radical oxidation and enzymatic pretreatment. The high pressure homogenization, fine grinding, high-strength ultrasound and high pressure microfluidization techniques are used to prepare cellulose nanofibrils. This review's objective is to offer a theoretical framework for the economical and environmentally responsible manufacturing of nanocellulose.