The plant cell wall consists of cellulose nanofibers or cellulose microfibril bundles. Since nanofibers are tight bundles of semi-crystalline extended cellulose chains, their thermal expansion is as low as that of quartz, while 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 the earth. Recently, due to the risk of depletion of oil reserves and global environmental concerns, the effective extraction of cellulose nanofibers and their utilizations have gained increasing attention. This review summarizes the research works on the production of nanofibers from plant resources and their application to design polymer composites.