Cellulose nanofibers (CNFs) have been widely used as functional fillers in polymer composites. In the present study, polymer composite nanofibrous nonwoven air filters containing CNFs were prepared, and the additive effect of CNFs on their performance was then investigated. CNFs with an average diameter of approximately 25-37 nm were obtained by water-jet nanofibrillation of pulps from dominant tree species in Japan, Japanese cedar, Japanese white birch, and bamboo. The filters were prepared by coating polymer/CNF composite nanofibers on polyester nonwoven substrates by Zetta spinning from poly(ethersulfone) (PES)/CNF composite solutions. The average diameter of the prepared composite nanofibers ranged from 67 to 89 nm. Aerosol filtration tests clearly indicate that the addition of a small amount of CNFs (similar to 0.8 wt %) to the PES nanofibers improves the filtration efficiency without reducing the transport properties. The maximum quality factor of PM0.4 at a face velocity of 5.3 cm/s is similar to 0.08 Pa-1, verifying the superior quality of the prepared composite nanofibrous filters. Our transmission electron microscopy observation and nitrogen gas and water vapor adsorption experiments suggest that the improvement in filtration efficiency occurs due to the surface functions of the added CNFs. These findings indicate that CNFs are a promising green filler for improving air filtration performance without the chemical modification of polymeric nanofibers.