The traditional method to isolate cellulose nanocrystals (CNCs) is to subject cellulosic materials to strong acid hydrolysis by mineral acids, which usually causes problems such as corrosion of equipment, the need for large amounts of water, the difficulty of acid recovery, and over-degradation of cellulose. Thus, a green and sustainable approach for the preparation of CNCs was developed where mild acid hydrolysis with diluted oxalic acid was used. The reaction kinetics of different preparation parameters, such as reaction temperature, oxalic acid dose, addition of HCl, and reaction time were thoroughly investigated. A high yield of up to 85 % was achieved by mild oxalic acid hydrolysis in comparison to the yield of 35 % using the most common approach with sulfuric acid hydrolysis. The CNCs from the above approach have a high thermal stability, that is, a maximum thermal degradation temperature of 350 °C in comparison to 200 °C when sulfuric acid hydrolysis was used. Importantly, oxalic acid solutions were readily recovered, and exhibited consistently high performance in several continuous runs of reaction. The hydrolysates contained mostly monomeric sugars, which could be further utilized for chemical or biofuel production.