The plant cell wall consists of a cellulose nanofiber or a cellulose microfibril bundle. Since nanofibers are bundles of semi-crystalline extended cellulose chains, their thermal expansion is as low as that of quartz whilst their strength is five times that of steel. However, the industrial utilization of nanofibers has been limited despite their being the most abundant biomass resource on earth. Recently, due to the risk of depletion of oil reserves and global environmental concern, the extraction of cellulose nanofibers and their utilizations has gained increasing attention. This review summarizes the research works on the production of nanofibers from plant resources and their application in polymer composites.