Esterified cellulose nanocrystals (E-CNCs) are cellulose derivatives that could be applied in biomedical and chemical industries. E-CNCs were prepared with cellulose pulp using a mixture of 17.5 M acetic and 18.4 M sulfuric acid with the aid of ultrasonication. The effects of esterification time (3-7 h), ultrasonication time (with a frequency of 40 kHz, 0 to 6 h) and temperature (68-75 degrees C) on the yield and degree of substitution (DS) of E-CNCs were evaluated. The sample obtained without ultrasonication had the lowest yield and DS value of 48.16% and 0.22, respectively, whereas ultrasonication for 5 h at 70 degrees C resulted in a yield of 85.38% and a DS value of 0.46. Characterization indicated the successful esterification of hydroxyl groups of cellulose, and the width of rod-shaped E-CNCs was from 10 to 100 nm. The study provides a simple and convenient method to manufacture E-CNCs. (C) 2012 Elsevier Ltd. All rights reserved.