Producing nanocellulose from lignocellulosic plants is difficult but can be achieved with microwave-assisted treatment. However, the changes in dimensions during the process have not been investigated thoroughly. In this study, kenaf bast was used to produce cellulose nanofibers using microwave, chemical, and ultrasonic treatments. Fiber sizes were monitored throughout the experiment using an optical microscope and scanning electron microscopy (SEM). The kenaf single fiber cells were also isolated and measured. The results showed that the duration of microwave treatment notably influenced the destruction of kenaf fibers, and the concentration of NaOH in the chemical treatment had only a limited effect on the reduction of kenaf particle size. Both the microwave and chemical treatments were able to destruct the kenaf fibers longitudinally, and the ultrasonic treatment was able to reduce cellulose particles from micro-size to nano-size.