Ultrasonic treatment is useful for the isolation of cellulose nanocrystals (CNCs). However, the effectsof ultrasonic treatment on the structure and properties of CNCs have not been fully understood. Westatistically analyzed the variations in yields and dimensions of CNCs prepared using acid hydrolysis, withor without ultrasonic treatment, and illustrated these variations by investigating the CNC structures. Theresults demonstrated that ultrasonic treatment promoted an increase in yields only for short hydrolysistimes, while resulting in CNCs with shorter lengths and narrower dimension distributions. Furthermore, aprolonged acid attack under ultrasonic treatment caused a partial dissociation of cellulose hydrogen bondnetworks in the CNCs, ultimately resulting in the delamination and disorder of the cellulose crystallinestructure, thus leading to a decrease in the width and thickness of the CNCs. This work provides moreinsights into the mechanisms of ultrasonic treatment on the structure and properties of CNCs.