The proteinaceous inhibitor of starch phosphorylase (SP) isolated from the root of sweet potato (Ipomoea batatas [L.] Lam.) had been identified as a β-amylase (BA).For clarifying ambiguities about the physiological roles played by SP and BA, it seems essential to elucidate the molecular mechanism of SP inhibited by BA. One of the possible molecular mechanisms is an end-products inhibition producted by BA. Therefore,the purpose of this study was to demonstrate the inhibitory possibitory of end-products produced by BA in the mechanism of SP inhibited by BA used by enzyme kinetics methods. The results indicated that SP had separate binding sites for the two substrates, starch and glucose-1-phosphate(G1-P), and the inhibition of SP by maltose resembling BA was competitive against starch. Therefore, the binding site of either BA or maltose to SP is different from that of G1-P. Although the inhibitory pattern of maltose was similar to BA, it could not inhibit SP unless the theoretical maximum concentration of maltose was attained by β- amylolysis in a 1% soluble starch. Moreover, the binding site of β-limit-dextrin, another end product of BA catalyzation, to SP was different from that of starch and its inhibitory pattern did not match with that of BA. To sum up, neither maltose nor β-limit-dextrin was the major cause forinhibition of SP by BA.