In recent years, cellulose nanofibers have been extensively studied in several research fields due to their renewability, availability and other remarkable properties. Several methods of cellulose nanofibers obtaining from different feedstock have been reported in the bibliography. The most common obtaining systems are based on the use of mechanical treatment such as grinding and homogenization. In this work, the production of cellulose nanofibers from recycled cellulose pulp using ultrasound technology was studied. The effect of different sonication times on nanofiber size was evaluated. The properties of the paper sheet formed with different percentages of cellulose fibers and nanofibers were determined. The obtained results showed that the paper sheet properties were improved by the nanofibers addition.