Vulcanized fiber is an all-cellulose material made from cotton and/or wood cellulose after zinc chloride treatment. This material was invented in the UK in the mid-19th century and is used because of its excellent characteristics, such as impact resistance and electrical insulation. Recent research revealed that the vulcanized fiber strength can be attributed to the chemically defibrillated cellulose nanofibers. In this report, we describe the history and structural characteristics of vulcanized fibers and introduce a new aspect of zinc chloride treatment.