A new method has been developed for acetylating cellulose using acetic acid. It has been found that cellulose is easily acetylated by acetic acid (AcOH) with the system of pyridine (Py) containing p-toluenesulfonyl chloride (TsCl) or Methanesulfonyl chloride (MsCl). Especially, rayon was acetylated to about 3.0 of D. S. within 2hr at 50°C by this system. The ratio of TsCl and AcOH was found to be an important factor for this reaction. When the equimolar ratio of TsCl and AcOH was used, the reaction proceeded most rapidly. Infrared spectra of the cellulose acetates obtained were also investigated.