Bast fibers of paper mulberry ( Broussonetia Kazinoki Sieb) were cooked with a weak alkaline salt, potassium carbonate which has been known as a major inorganic component of the traditional lye, and its cooking characteristics were investigated . The bast fiber was easily cooked by potassium carbonate. The pulp yield was rapidly decreased up to 20-30 mol of potassium carbonate, but the Kappa number was slowly decreased with increasing of potassium carbonate. The potassium carbonate pulps were easily defibered at low cooking chemical charge of 25mm on and high pulp yield of about 80%. These results were confirmed that pectin was easily removed during the potassium carbonate cooking. In contrast, when sodium carbonate was used as a cooling agent, the bast fiber was only partially defibered. Thus, sodium carbonate was a less effective cooling chemical of the bast fiber. The results of this experiment indicated that potassium carbonate could not only be used as a good cooling agent of bast fiber, but also as an alternative agent of sodium hydroxide.