AbstractUltralong cellulose nanoﬁbers with extremely high aspect ratio were successfully manufactured from waste corrugated paper pulp through a series of chemical treatments combined with grinding, ultrasonication, and centrifugation. SEM images revealed that the prepared cellulose nanofibers exhibited a uniform width ranging from 30 to 100 nm and a web-like network structure. The nanopaper was produced by filtration and oven drying using the obtained cellulose nanofibrils suspension. An interesting phenomenon occurred, namely that the nanopaper formed in multilayered nanofibrous flakes, which can be seen in the SEM image of the nanopaper cross section. The nanopaper derived from waste corrugated paper presented high tensile properties, with a tensile strength of 135 MPa and a tensile modulus of 6.67 GPa, which was approximately 10 times higher than the untreated waste corrugated paper. The obtained nanopaper also exhibited high transmittance of 85.2% at 600 nm wavelength and low thermal expansion of 16.2 ppm/K. The high performance nanopaper seems to be a strong candidate for fabricating optical electronics, solar cells, and panel sensors.