Cellulose nanofiber composites cause less damage to the environment, but poor strength is a major drawback. Therefore we focused on the orientation of nanofibers as a way to strengthen the composite material. By unidirectionally orienting fibers, mechanical properties can be improved as reported in previous studies. This study examined the effect on mechanical properties caused by cellulose nanofibers extension. Specimens consisted of cellulose nanofiber and polyvinyl alcohol. The experimental set-up consisted of a modified vice with aluminum jigs to fix the specimens for extension. Wet specimens were extended several times (1, 5 and 10 times) by 2% strain at every extension cycle. Due to large extension exerted at once broke the specimens, a cyclical extension was carried out instead. As a result, 5 times-extended specimens reached the highest tensile strength and Young's modulus at 157.3 MPa and 18.2 GPa, respectively. Compared to non-extended specimens, tensile strength increased 32% and Young's modulus increased 16%. Consequently, cyclical extension of specimens led to the nanofibers orientation, producing stronger composites without causing fracture.