Sweet potato β-amylase is a tetrameric enzyme consisting of four identical polypeptide chains with a molecular weight of 5.6×10 exp (4), though most of the other β-amylases are monomeric enzymes. But, the relationship between subunit structure and catalytic function of the enzyme is not known. This study was done to know what the function of the subunit structure of the enzyme is. We obtained the monomer from the enzyme by the treatment of SDS, alkali pH buffer and urea. But the monomer had not activity. We tried to prepare the active monomer from the enzyme by the modification with periodate-oxidized soluble starch. In the result, we succeeded in isolating an active monomer as an oxidized soluble starch-conjugated form. The active monomer had 57% of the original activity, 13.2% of the sugar and the molecular weight was estimated to be 6.4×10 exp (4). This results suggest that the tetrameric form of the enzyme is a most stable one and exists in nature, and the subunit structure of the enzyme plays an important role in stabilization but not catalytic function.